High Speed Spindle Design and Construction

Engineering Research Center for Net Shape Manufacturing

Article From: 8/15/1998 Modern Machine Shop,

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Introduction

A high speed spindle that will be used in a metal cutting machine tool must be designed to provide the required performance features. The major performance features include:

  • Desired Spindle Power, Peak and Continuous
  • Maximum Spindle Load, Axial and Radial
  • Maximum Spindle
  • Speed Allowed
  • Tooling Style, Size and Capacity for ATC
  • Belt Driven or Integral Motor-Spindle Design


Although these criteria may seem obvious, for the spindle designer, they represent a wide array of needs that are quite difficult to meet and optimize, in one design. As we will discover, many of the criteria are contradictory to one another, and eventually a compromise must be chosen to provide the best design.

The machine tool, also, will present design constraints to the spindle. The amount of available space in the head, cost considerations, complexity and market demands will affect the ultimate spindle design. Cost will have a significant impact on the final spindle design. A very sophisticated and capable spindle design will not be acceptable on a low-cost machine tool. Consequently, an advanced machine tool design can justify the higher cost of a more capable and complex spindle package. In fact, a fast and accurate machine tool will demand a reliable high-speed spindle system.

This paper will give a brief overview of the major components required to comprise a high speed milling spindle design. Emphasis will be on commercially available components, that are available at reasonable cost, and most commonly used today on existing machine tools. Future trends will also be mentioned.

In addition to the high speed spindle system design, maintenance and reliability issues will also be discussed.

High Seed Spindle Design: Major Component List

The major components required for a high speed milling spindle design include:

  • Spindle Style; Belt Driven or Integral Motor-Spindle
  • Spindle Bearings; Type, Quantity, Mounting, and Lubrication Method
  • Spindle Motor, Belt-Type, Motor-Spindle, Capacity, Size
  • Spindle Shaft; Including Tool Retention Drawbar and Tooling System Used
  • Spindle Housing; Size, Mounting Style, Capacity


Each of these components will be discussed, with emphasis on selection criteria and effectiveness for a given machine tool specification. The machine tool we will assume is a modern CNC machining center with automatic tool changing ability (ATC).

Spindle Style: Belt-Driven or Integral Motor-Spindle

The first decision which must be made is if a belt-driven spindle or integral motor-spindle design will be required This must be determined by evaluating the requirements of the machine tool, including the maximum speed, power and stiffness required, Also, cost is an important factor, as a belt driven spindle generally is a lower cost solution than an integral motor-spindle.

Belt-Driven Spindle Design (click here to see graphic)

A belt-driven high speed spindle is quite similar in design to a conventional speed spindle design, with some noticeable differences. A typical belt driven spindle assembly consists of the spindle shaft, held with a bearing system and supported by the spindle housing. The spindle shaft incorporates the tooling system, including the tool taper, drawbar mechanism and tool release system. The mechanism that provides the force to provide a tool unclamp is most usually externally mounted

Power and rotation are supplied to this spindle by an external motor. The motor is mounted adjacent to the spindle, and the torque is transmitted to the spindle shaft by means of a cogged or V-belt. The power, torque and speed of the spindle will therefore depend upon the characteristics of the driving motor, and the belt ratio used between the motor and the spindle.

The principal advantages of the belt-driven spindle design are as follows:

Reasonable Cost: As the spindle itself is comprised of a few basic parts, the cost is relatively low, when compared to alternative solutions.
Wide Variety Of Spindle Characteristics: As the spindle power, torque and speed are dependent upon the driving motor, to a large degree, the final specifications can be modified for a particular application by using a different motor or belt ratio. In some cases, gears are also used to provide multiple speed ranges in addition to the fixed belt ratio.
High Power and Torque Possible: The spindle motor is mounted externally from the actual spindle shaft, and therefore it is often possible to use a very large motor. A large motor, particularly one of large diameter, can provide very high torque and high power for spindle use. This is much more difficult in an integral motor-spindle design, as available space is always limited.
However, there are also some limitations of a belt-driven spindle design, particularly when a high speed spindle is required:

Maximum Speed is Limited: A belt-driven spindle will be limited in maximum rotational speed due to several factors. The mechanical connection which transmits the torque to the spindle shaft, the belt and pulley system, are limited in maximum operating speed. If a poly V-belt system is used, high rotational speeds on the pulleys tend to stretch and disengage the belts, reducing their contact and ability to transmit torque. Cogged belts eliminate the slipping problem, however, at higher speeds these belts produce unacceptable levels of vibration. Gears are very limited in maximum speed, and will also produce high levels of vibration and heat if operated at very high speeds.
Belts Utilize Bearing Load Capacity: In order to be able to transmit the necessary torque, belt-driven spindles utilize a belt and pulley connection on the end of the spindle shaft. The required tensioning of these belts will exert a constant radial force on the rear spindle shaft bearing set. As the power and speed of the spindle increase, the applied tension and consequent force will increase, using up much of the available radial loading capacity of the bearings. And, substituting larger bearings, or adding additional bearing sets will not be feasible, as these methods will only further reduce the spindle abilities to reach high rotational speeds.

Summary

Therefore, it is evident that a belt-driven high speed spindle will be limited to certain applications. Typically, belt-driven spindles will be used up to maximum rotational speed of 12,000 - 15, 000 RPM. To accomplish this, other means must be used to allow the higher speeds, including different bearings types, setups, or bearing lubrication. These will be further discussed in subsequent sections of this report, as they are similar to methods used in motor-spindles. Power for this type of spindle may reach as high as 30 HP, however, it is sometimes difficult to provide high torque at the top speed This will depend very much on the driving motor characteristics.

Integral Motor-Spindle Design(click here to see graphic)

The integral motor-spindle does not rely upon an external motor to provide torque and power. The motor is included as an integral part of the spindle shaft and housing assembly. This allows the spindle to rotate at higher speeds as a complete unit, without the additional limitations of belts or gears.

In general, a complete motor-spindle is comprised of the spindle shaft, including motor element, and tooling system. The spindle shaft is held in position by a set of high precision bearings. The bearings require a lubrication method, such as grease or oil. The spindle shaft then will rotate up to the maximum speed, and exhibit the power characteristics of the motor type that is used. The selection of a particular component will, of course, depend upon the requirements of the machine tool. Also, compromises must be made in order to provide the best combination of speed, power, stiffness and load capacity. The following sections will describe in more detail the design and selection criteria used for the major components of a high speed motor-spindle.

Spindle Bearings: Type, Quantity, Mounting, and Lubrication Method

One of the most critical components of any high speed spindle design is the bearing system. Our design requirements state that the spindle must provide high rotational speed, transfer torque and power to the cutting tool, and be capable of reasonable loading and life. The bearing type used must be consistent with these demands, or the spindle will not perform.
High precision bearings are available today from a variety of manufacturers worldwide. The type of bearings available for high speed spindles include roller, tapered roller and angular contact ball bearings. The selection criteria of which type to use will depend upon the spindle specifications, as each will have an impact or impact upon the bearing selection, as the following table explains.

Requirement
High Speed
High Stiffness
Axial Loading
Radial Loading
High Accuracy
Best Bearing Type
Small Angular Contact
Large Roller
High Contact Angle
Low Contact Angle
ABEC 9, High Preload
Design Impact
Small Shaft, Low Power
Low Speed, Large Shaft
Lower Speed
Higher Speed
Expensive, Low Speed

 As you can see, there are many factors that determine the final decision. A spindle that is desired to have the highest speed will not have the maximum stiffness possible, and, the spindle with the highest stiffness cannot run at high speeds without sacrificing bearing life. So, as designers, compromises must be made in order to arrive at a final design that will offer the compromise.

Angular Contact Ball Bearings vs. Tapered Roller Bearings

Angular contact bearings are most commonly used today in very high speed spindle designs. This is due to the fact that angular contact ball bearings provide the precision, load carrying capacity and speed required for metal cutting spindles. In some. cases, tapered roller bearings are used, due to their higher load capacity and greater stiffness over ball bearings. However, tapered roller bearings do not allow the high speeds required by many spindles.
Angular contact ball bearings utilize a number of precision balls fitted into a precision steel race. They are designed to provide both axial and radial load carrying capacity, when properly pre-loaded.

An important concept to consider is the maximum speed a bearing, and ultimately, the spindle will be able to achieve. This is determined, theoretically, by considering the type of bearing, lube method, pre-load, loading, etc. In practice, a bench mark is used, referred to as the dN number. The dN number is derived by multiplying the bearing bore diameter by the speed in RPM. For a high speed spindle, utilizing ball bearings, dN numbers up to 1,500,000 are possible.

Angular contact ball bearings are manufactured to a specification that includes a contact angle. The contact angle is the nominal angle between the ball-to-race contact line and a plane through the ball centers, perpendicular to the bearing axis. (See Fig. 3) The contact angle determines the ratio of axial to radial loading possible, with radial loading being the primary benefit. Typically, contact angles of 12º, 15º, and 25º are available. The lower the contact angle, the greater the radial load carrying capacity, the higher the contact angle the higher the axial loading capacity will be. Therefore, it may be desirable to use a bearing with a contact angle of 25º for a spindle that will be used primarily for drilling, and a contact angle of 15º for a spindle that will primarily be used for milling.

Contact Angle (click here to see graphic)

All precision bearings are manufactured to a tolerance standard. The most commonly used standard in the United States is the ABEC standard (America Bearing Engineers Committee) This standardization has been accepted by the American National Standards Institute (ANSI) conform essentially with the equivalent standards of the International Organization for Standardization (ISO). ABEC standards define tolerances for major bearing dimensions and characteristics. They are divided into mounting dimensions (bore, I.D. and width) and bearing geometry. Accuracy ratings range from a low of ABEC 1, for a general purpose bearing, to a high of ABEC 9, which describes a high precision bearing suitable for use in a high speed spindle. Typically, spindle bearings are manufactured with geometry accuracy of ABEC 9, to provide minimum runout and rotational accuracy. Bore, O.D., and width are manufactured to ABEC 7, which allows for a more reasonable fitting and installation.

Angular contact ball bearings are available with a choice of pre-loading magnitude, typically designated as light, medium and heavy. Light pre-loaded bearings are designed to allow maximum speed and less stiffness. Heavy pre-loading allows less speed, but higher stiffness. Light pre-loaded bearings are often used for very high speed applications, where cutting loads are also light, and top RPM is needed.

In order to provide the required load carrying capacity for a metal cutting machine tool spindle, several angular contact ball bearings are used together. In this way, the bearings can share the loads seen, and increase the overall spindle stiffness. The bearings can be stacked in several ways, depending upon the desired characteristics. Angular contact ball bearings must be pre-loaded in order to provide axial and radial capabilities. One simple way to do this is to mount the bearings in groups of two or three, such that the pre-load is correctly applied to the he bearings. This is possible by using duplex bearings, which are manufactured for this purpose. The inner or outer bearing races are ground, such that when clamped together the bearings will have the proper amount of pre-load

Face to Face/DF X Configuration Bearing Mounting (click here to see graphic)

If the bearings are mounted face to face, this is referred to as "Face-to Face" or "X" configuration. In this configuration, the outer races are relieved When the outer races are clamped together, the relief clearance is eliminated, resulting in the correct pre-load In this mounting technique is not very common, however, it does provide the proper pre-loading, such that the bearing pair is capable of withstanding both axial and radial loading.

Back to Back/DB/O Configuration Bearing Mounting (click here to see graphic)

The most common technique used is back to back, "O", or "DB " mounting. In this configuration, the inner races are relieved. When the inner races are clamped together, the relief clearance is eliminated, resulting in the correct pre-load This configuration is suited for most applications and provides good accuracy and rigidity.

Tandem/DT Configuration Bearing Mounting (click here to see graphic)

Generally, a combination of mounting techniques is used in a spindle assembly. In many cases, two or three bearings are placed near the spindle nose, with a pair mounted near the rear of the spindle shaft. This mounting is known as "Tandem" or "DT". Tandem mounting does not allow forces in both directions, unless another pair of bearings are used on the spindle shaft, facing in the opposite direction. To increase moment loading capacity and spindle performance, spacers are used to separate the bearing sets.

This is generally the case in most motor-spindle designs. The spindle designer will use two or three bearings in the front in a tandem setup. At the rear of the spindle shaft, another bearing pair, of equal size or smaller, will be used in a tandem arrangement as well. Together, the bearing sets form an overall "DB" or Back-to Back setup. The spindle shaft and spindle housing locate the bearings.

Motor-Spindle with Tamdem Bearing Pairs and Spring Pre-Loading(click here to see graphic)

Motor-spindles experience temperature increases due to bearing heat and motor losses. This heat results in thermal growth of the spindle shaft. As illustrated, a tandem pair of bearings, used both in the front and rear of a spindle will initially have affixed pre-load based upon the location of the bearings in reference to the spindle housing. When subjected to heating, the spindle shaft will grow in length. This change in dimension will be seen by the bearings as an increase in pre-load, as the inner race is forced into the bearing. This is most undesirable, and can cause rapid failure of the bearings.

To compensate for this change, it is often necessary to mount the rear spindle bearings in a floating housing, with springs. The floating housing is mounted in a precision bore, or ballcage, that is free to move in an axial direction only. Springs are used to provide a constant pre-loading force against the spindle shaft in the axial direction. As the spindle grows due to thermal expansion, the rear bearings are free to move also. In this way, the pre-load seen by the bearings does not change, and is maintained by the force exerted by the springs. This technique is used for high speed milling spindles and grinding spindles, and does add a degree of cost and complexity to the spindle system.

Bearing Construction: Hybrid Ceramic (click here to see graphic)
A recent development in bearing technology is the use of ceramic (silicon nitride) material for the production of precision balls. The ceramic balls, when used in an angular contact ball bearing, offer distinct advantages over typical bearing steel balls:

The ceramic balls have 60% less mass than steel balls: This is significant because as a ball bearing is operating, particularly at high rotational speeds, centrifugal forces push the balls to the outer race, and even begin to deform the shape of the ball. This deformation leads to rapid wear and bearing deterioration. Ceramic balls, with less mass, will not be affected as much at the same speed. In fact, the use of ceramic balls allows up to 30% higher speed for a given ball bearing size, without sacrificing any bearing life.

Ceramic Balls Do Not React with The Steel Raceways: One of the most prominent mechanisms of bearing failure is surface wear created by microscopic "cold welding " of the ball material to the raceway. The cold welds actually break as the bearing rotates, creating surface roughness which leads to heat generation and bearing failure. The ceramic material drastically reduces this mechanism, resulting in longer bearing life.

Ceramic Ball Bearings Operate At Lower Temperatures: Due to the nearly perfect roundness of the ceramic balls, hybrid ceramic bearings operate at much lower temperatures than steel ball bearings. This results in longer life for the bearing lubricant. Ceramic Bearings Operate at Much Lower Vibration Levels: Tests have shown that spindle utilizing hybrid ceramic bearings exhibit higher rigidity and have higher natural frequencies, making them less sensitive to vibration.

Bearing Lubrication Methods

Angular contact ball bearings require some form of lubrication to operate properly. The function of lubrication is to provide a microscopic film between the rolling elements, to prevent abrasion and skidding. In addition, lubrication protects the surfaces from corrosion, and protects the area from particle contamination.
The most common type of lubricant is grease. Grease provides the most simple method of lubrication. The grease is injected into the space between the balls and the races, and is permanent. It therefore requires minimal maintenance, and represents little if any cost. Grease lubrication, however, does have limitations. Grease packed spindles generally are not run above dN values of 850,000 for continuous operation. As speeds increase, operating temperatures increase, and begin to break down the grease. Most grease is rated to operate at temperatures under 300° F. The grease type that can tolerate high speeds generally has a ester oil base, and Barium complex thickener. An example of this would be Kluber Isoflex NBU-15. With regard to the quantity of grease, more is not better. Excessive grease can cause heating due to churning, which will cause the grease to deteriorate. Approximately 20% to 30% of the open area between the races should be filled. Following the grease fill, a careful run-in period is required, to fully distribute the grease within the bearing.

In general, high speed spindles that utilize grease lubrication do not allow for replacement of the grease between bearing replacements. During a bearing replacement, clean grease is carefully injected into the bearing. Positive air overpressure is typically used to prevent contamination from entering the bearing, which could lead to rapid bearing failure.

Table 31. Typical Grease Lubricants

 

Bardon Code Designation Base Oil Thickener Operating Temp. Range °F Maximum dN Comments
G-2 Exxon Beacon 325 Diester Lithium -65 to 250 400,000 Good anti-corrosion, low torque.
G-4 NYE Rheolube 757SSG Petroleum Sodium -40 to 200 650,000 Anti-oxidation additives, machine tool spindle grease. Not water resistant.
G-6 Exxon Andok C Petroleum Sodium -20 to 250 650,000 Good for high speed, low grease migration. Not water resistant.
G-12 Chevron SRI-2 Petroleum Polyurea -20 to 300 400,000 General purpose, moderate speed, water resistant.
G-18 NYE Rhoetemp 500 Ester and Petroleum Sodium -50 to 350 500,000 For high temperature, high speed. Not water resistant.
G-32 Supermill A72832 Diester Lithium -100 to 250 400,000 MIL-G-23827, EP, anti-corrosiion additives.
G-33 Mobil 28 Synthetic hydrocarbon Clay -80 to 350 400,000 MIL-G-81322, DOD-G-24508, wide temperature range.
G-35 DuPont Krytox 240 AB Perfluoro-
alkylpolyether
Tetrafluoro-
ethylene-
telomer
-40 to 450 400,000 Excellent thermal oxidative stability, does not creep, water resistant and chemically inert.
G-42 NYE Rheolube 350-SBG-2 Petroleum Sodium /Calcium -30 to 250+ 650,000 Spindle bearing grease for normal temperatures and maximum life at high speed.
G-44 Braycote 601 Perfluorinated Polyether Tetrafluoro- ethylenete-
lomer
-100 to 500+ 400,000 Excellent thermal and oxidative stability, does not creep, water resistant and chemically inert.
G-46 Kluber Isoflex NBU-15 Ester Barium Complex -40 to 250+ 850,000 Spindle bearing grease for maximum speeds, moderate loads.
G-47 Kluber Asonic GLY32 Ester/ synthetic Hydrocarbon Lithium -60 to 300 600,000 Quiet running spindle bearing grease for moderate speeds and loads.
G-50 Kluber Isoflex Super LDS 18 Ester/mineral Lithium -60 to 250 850,000 Spindle bearing grease for maximum speed and moderate loads.
G-51 Mobilith SHC 15 Synthetic hydrocarbon Lithium -60 to 450 500,000 General purpose, moderate speed, water resistant.
Grease Types Used in High Speed Spindle Bearings (Courtesy Barden Co.)

 

Oil Lubrication Techniques (click here to see graphic)

In many cases, particularly when high rotational speed is required, grease bearing lubrication is not sufficient. Oil is then used as a lubricant, and delivered in a variety of ways. As previously mentioned, grease can support bearing speed up to a dN value of 850, 000. Oil lubrication can support speeds up to dN of 1,500, 000.

One common method of oil lubrication is oil mist. An oil storage tank is used, and compressed air is mixed with the oil. This creates oil droplets that are carried by the air flow to the bearing area. The major benefits of oil mist are good supply of lubricant, simple, and in addition to lubricating the bearing the oil mist also cleans and cools the bearings. This system is best applied to spindles having high speeds and relatively light loads. Oil mist is somewhat difficult to measure and control, so if the quantity of oil delivered to the bearings must quite accurate, oil mist may not be the best system to use.

Another common method of oil lubrication is oil jet. Oil jet utilizes a high pressure pump that delivers oil directly into the bearing race. This system is suitable for spindles that must tolerate high loads, high speeds, and high temperatures. Care must be taken to insure that the oil can be quickly routed through the bearing, or oil churning will develop. This system requires a complex pump, storage tank and temperature control system, however, it is sometimes necessary to support very high performance spindles. Another system, pulsed oil-air, injects oil in very small quantities with compressed air, into the bearing cavity. The frequency of injection may be related to the spindle operation, or simply on a timed basis.

There are many oils available on the market today that are effective for use in high speed bearings. The following chart, courtesy of Barden, Co., illustrates some of the types and specifications available.

Table 30: Typical Oil Lubricants S


Barden Code Designation Base Oil Operating Temp. Range °F Maximum dN Comments
0-9 Exxon instrument oil Petroleum -65 to 150 1,500,000* Anti-oxidation, anti-corrosion E.P. additives.
0-11 Winsorlube L-245X Diester -65 to 175 1,500,000* Attacks paint, neoprene, anti-corrosion additives. MIL-L-6085.
0-14 Exxon Turbo Oil #2389 Diester -65 to 350 1,500,000* Anti-oxidation, additives, MIL-L-7808.
0-17 Nyosil M-20 Silicone -100 to 350 200,000 Low surface tension, tends to migrate, MIL-S-81087 Type 1.
0-28 Mobil SHC224 Synthetic hydrocarbon -65 to 350 1,500,000* Good heat stability, low volatility.
0-49 Exxon Turbo Oil #2380 Diester -65 to 350 1,500,000* Anti-oxidation additives, MIL-L-23699
0-50 NYE Synthetic 181B Synthetic hydrocarbon -40 to 300 1,500,000* Good heat stability, low volatility.
0-59 Bray Micronic 815Z Perfluorinated polyether -100 to 500 400,000 Low surface tension, but does not migrate.
0-62 DuPont Krytox 1506 Fluorocarbons -60 to 550 400,000 Low surface tension, but does not migrate.
High Speed Spindle Bearing Lubricating Oils (Courtesy Barden Co.)

 

Summary

Bearing Lubrication is a critical component in the complete high speed spindle system. Depending upon bearing size, type, and speed, bearing lubrication may be permanent grease packed or some type of oil system. The maintenance of the lubrication system is vital, and must be closely monitored to insure that proper bearing conditions are maintained. Oil mist, oil jet, and pulsed oil-air systems require that clean, dry and continuous air be supplied. Also, use of the correct type, quantity, and cleanliness of lubricating oil is critical.
Bearing Life Calculation (click here to see graph)
All bearings will have a useful life, defined as operation time until the bearing specifications are lost, or a complete failure of the bearing occurs. The most common cause of bearing failure is fatigue, which causes the races to become rough, leading to heating, and eventual mechanical failure. Bearing life, in general is affected by the following parameters:

Bearing Loads, Axial and Radial
Vibration Levels
Quality and Quantity of Lubrication
Maximum Speed
Average Bearing Temperature
Bearing life is typically expressed as L10 life. This is defined as the minimum life in revolutions for 90% of a typical group of apparently identical bearings. The calculation for this is expressed as follows:


 
Where:
C33 = Basic Dynamic Load Rating
K = Factor for Multiple Bearings
P = Equvalent Radial Load

And:

P = XR + YT
R = Radial Load
T = Thrust Load
X = Radial Load factor relating to contact angle
Y = Axial Load factor depending upon contact angle, T, and ball complement.

If we take into consideration time, a life can be expressed in terms of hours of operation:

In general, bearing life, and ultimately spindle life, will depend upon many factors, including speeds, loading, lubrication and bearing size. Computer models are often used to forecast life, however, a typical spindle bearing life for very high speed operation should be in the range of 5000 - 7000 hours, assuming the spindle is not crashed or mis-used. This following graph illustrates theoretical bearing life based on speed and bearing loading factors. (Courtesy SNFA)

Spindle Motor Design (click here to see graphic)

Integral motor spindles must utilize an electrical motor as part of the rotor shaft. Therefore, the motor size and capacity will depend strongly upon the available space. As we have discussed earlier, bearing size is critical in a high speed spindle design, so the motor shaft will affect the bearing size that can be used. The bearing size also affects the loading capability, stiffness, and maximum speed, so the motor characteristics must match the bearing capability.

The most common type of motor used in high speed motor spindles is an AC induction motor. In this design, the rotor is attached to the spindle shaft, either with an adhesive or thermal clamping. The rotor and stator, the winding in which the rotor revolves, are generally provided by a motor or drive supplier. The rotor is attached to the shaft during assembly. Following this, the bearings are mounted to the front and rear of the shaft, and the shaft is then fitted into the spindle housing.

The spindle shaft is quite important, as it must transfer the power from the motor to the cutting tool. the shaft must locate and support the bearings, and contain the complete tooling system as well. One important design consideration for the shaft is bending. During high speed operation, the shaft will exhibit bending characteristics. The frequency at which the shaft will bend depends on the diameter and length of the spindle shaft. It is often tempting to design a very long spindle shaft, as this increases the load carrying capacity of the spindle and allows for a more powerful motor. However, care must be taken as the spindle grows in length, the first bending mode will approach frequencies in the operating zone. This is not tolerable for spindle operation, and must be resolved by either re-designing the shaft with a larger diameter (bearings will be larger and slower!) or decreasing the shaft length. The following diagram illustrates the bending modes of a spindle. (courtesy IBAG Zurich)

Spindle Motor Power and Torque (click here to see graphic)

AC induction motors exhibit power and torque curves determined somewhat by the winding design. However, due to limited space available, and centrifugal forces acting on the laminated rotor, power is related closely to speed

Spindle motors will generally provide constant torque up to the base speed, and constant horsepower after the base speed As power is a function of speed multiplied by torque, the following curves are typical. Conventional spindle heads multiply available torque by using mechanical components such as gears and pulleys.

Motor-spindles, however, must rely upon a single motor characteristic to provide the power and speed needed for machining across the full range of operation. The result is generally that the spindles are designed and intended to be used at or near full speed. Below this, as power falls off, little heavy machining is feasible.

Integral AC induction motors are typically three phase, requiring a special electronic drive to provide the electrical power source. The drive is a high frequency type, and provides a variable voltage and variable frequency to the spindle motor. The speed of an AC motor is determined by the following formula:


Speed (RPM) = (Frequency in Hz x 120) / (# of motor poles)

This would dictate that a two pole spindle motor, having a top speed of 30,000 RPM, would require a drive with the capability to provide full motor voltage at an output frequency of 500 Rz. If this motor was a four pole type, then a maximum frequency of 1000 Hz would be required.

Very high speed drives utilize an open-loop concept, providing voltage and current to the motor without any real-time feedback to close the velocity or position loop. Many drives, however, do use magnetic or optical feedback to the spindle drive. This is used to regulate speed, provide programmable positioning of the spindle shaft, and in some cases rigid tapping. Orientation is required for many tooling systems for ATC operation. DC brushless and Flux vector are examples of closed loop systems.

Spindle Air Seals and Labyrinth Designs (click here to see graphic)

High precision bearings are quite sensitive to external contamination. Chips, dust, dirt, coolant, and other foreign material will contaminate the bearing surfaces, resulting in pre-mature failure, particularly in grease packed bearings.

To protect against this condition, spindle designers utilize some type of seal to prevent contamination from entering the spindle. The most simple type is a positive air over-pressure. Compressed air is directed into the spindle housing, at low pressure. The air feeds outward to the front and rear of the spindle, providing a low flow of air. This flow prevents contamination from entering into the spindle.

This is particularly important for motor-spindles, due to a "chimney affect" which often occurs. As a motor-spindle operates, losses in the rotor will produce heat. As the only contact between the rotor and the spindle housing is through the bearings, the shaft will increase in temperature. When the spindle is stopped, the hot rotor will heat the adjacent air, which will rise. This movement of air, as in a chimney, will draw outside air into the spindle, often bringing contamination with it. This can be very damaging if the material being cut is graphite or carbon. A positive air over-pressure will protect the spindle from this effect.

One of the most vulnerable areas in a spindle is near the spindle nose. In this area, the front bearings are very close to the machining area, and subjected to the coolant splashing and chips. Therefore, it is important to provide an extra measure of care to protect the sensitive spindle bearings. Contact seals are not feasible, due to the high speeds. Instead, labyrinth seals are used A labyrinth seal is a non-contact sealing system comprised of a fixed and rotating part. Both parts have channels and grooves machined into them, so they fit together to form a series of passageways between the spindle bearing and the outside air. It is very difficult for a particle of dirt of coolant liquid to pass through a labyrinth seal. Labyrinth seals, used in conjunction with positive air over-pressure, provides very good protection for a high speed spindle.

Tool Retention System

A high speed spindle designed for use in a CNC machining center must be able to automatically change tools. This is done by incorporating a tooling system Common tooling systems include CAT, BT and ISO styles. More recently, a new DIN and ISO tooling standard has been developed with particular application for high speed, known as HSK.
This report will not attempt to compare the tooling styles, however, the CAT, BT, and ISO standards are questionable as tooling choices for very high-speed. As these tooling standards were developed prior to high speed cutting, the tolerances allowed do not always match the strict requirements of high speed machining. If one of these styles is used, accuracy, cleanliness, and most importantly balance are very critical issues to consider.

The spindle must provide a means to locate and clamp the toolholder. This is accomplished by machining a taper in one end of the spindle, manufactured to match the appropriate taper angle and diameter required by that tooling specification. In addition, a clamping mechanism must be provided to hold the toolholder in the taper during machining operations. This device, a drawbar, must provide sufficient pulling force to overcome all forces created by cutting that would tend to pull the tool out of the spindle. The most common technique used in drawbar construction is to stack belleville washers to create a long tension ring. The end of the drawbar grips the toolholder retention knob, and holds the toolholder in position in the taper. When a tool change must occur, a hydraulic or pneumatic cylinder compresses the drawbar, and the toolholder is released

With regard to spindle design, the drawbar presents some challenges. A drawbar is a movable device, and with each actuation the springs may end up in slightly different locations. This can create a balance problem, which could cause unwanted vibration at high speeds. To overcome this, drawbar components are manufactured to close tolerances, and guide bushings are used internally.

Also, as speeds increase, the holding force required also increases. It is not practical to increase the holding force by simply increasing the number of washers, as this would require that the spindle shaft be longer (remember bending modes?). It is also not always practical to increase the diameter of the washers, as this may require the shaft to be larger (larger bearings, lower speed!).

To satisfy the holding force requirement, mechanical locking systems are sometimes used. The drawbar uses belleville washers to pull the toolholder into the taper. Once seated, however, a mechanical locking system then is actuated. The locking components may be small balls or cams. After the locking mechanism is in place, all cutting forces are directed against the solid steel shaft, not against the belleville washers. This system provides very high holding force and rigidity, which is critical to the high speed cutting process.

As the spindle will be used with an ATC magazine, it is necessary to have electronic sensors or switches built-in to indicate to the control logic when a tool is clamped, unclamped, or missing. These signals must be derived from monitoring the position of the drawbar.

Spindle Housing

The spindle shaft and motor must be held in a housing. The housing may be an integral part of the machine tool, or it may be a cartridge housing. Many high speed spindle designs utilize a cartridge type housing, as this is the simplest to service, and the tolerances required for high speed are easier to obtain when the housing can be produced as a cylinder.
The primary function of a spindle housing is to locate the bearings. High precision bearings, being run at top dN values, must be located exactly in terms of geometry and size. In addition, the housing will provide the lubrication, air seal, cooling water or oil, and other utilities required by the spindle. If the spindle utilizes oil lubrication, the housing will include drilled passages to deliver the oil or oil mist to each bearing, and out of the bearing to a return line. A cooling liquid is often used to remove heat produced by the spindle motor stator, as this heat would affect the size and accuracy of the spindle as a complete unit.

The spindle housing is typically connected to the machine tool by means of a flange or attaching bracket. Care should be taken when handling any precision spindle. Crashes, dents, and other damage can affect the accuracy and bearing life.

Conclusions

A high speed spindle design must take into consideration the desired end result: the required power, speed, torque, tooling system used, accuracy, and life. From this design specification, the needed components can be selected including bearings, shaft design, motor, lubrication system, tooling style, drawbar system, housing and cooling system.
As we have seen, bearings will impact a spindle design to the greatest degree. High speed spindle designs most often run bearings systems up to the limit, in order to be the most productive. And, as integral motors are limited in maximum torque available, higher speeds will yield higher power. To reach these speeds, and maintain a reasonable life, precision bearings must be used, along with complex bearing lubrication systems. Oil jet or mist systems not only boost the speed of the bearings, they also provide cooling and cleaning functions as well. Maintenance is critical to the performance of precision bearing systems. Positive over-pressure and labyrinth air seals also should be used to protect the bearing environment.

In addition to the bearings, the spindle shaft design must be capable of providing a strong motor, suitable tooling retention system, and stiffness without developing bending problems. And, all rotating components must operate in a balanced condition.

The spindle housing must support and locate the bearings accurately, and provide the utilities needed by the spindle system. It must be robust and stiff, as the housing transfers all forces from the spindle to the machine tool.

In general, a high speed spindle design will be the result of many compromises. Bearing size and type will dictate maximum speeds possible. Increasing pre-loads and additional tandem bearings will increase stiffness, but speed will be sacrificed High power motors will not fit into the design envelope, and more complex drive systems are required. Higher speeds require higher precision tooling systems, better balance, and cleanliness to obtain the desired results. Shop discipline must be strict. Operators should be well trained and encouraged to learn more about the machine tool.

Future Trends

In the opinion of any spindle designer, the ultimate spindle would have the following characteristics:
Unlimited Speed
High Power
Long Life
Self-Balancing
Self-Diagnostic
As unattainable as these qualities may sound, they will be fulfilled in the future. High speeds can be accomplished through the use of magnetic or fluid bearings. These non-contact bearing systems will exhibit no mechanical wear, so their life will be very long. Electronic sensors will monitor all aspects of the spindle operation, including cutting loads. Imbalance can be compensated for as the spindle runs. Diagnostic information can be relayed to the CNC for action. Superconducting materials and new motor technologies will provide compact, high power motor system's that produce little heat. Thermal affects on the spindle shaft can be compensated for electronically.

References

The Barden Corporation
200 Park Avenue
Danbury, CT 06813
T(203) 744-2211
F (203) 744-3756

SNFA Route de la Glane 143/B
Case Postale 34
CH 1752 Villars sur Glane
T (037) 24-07-66
F (037) 24-06-14

SKF Specialty Products
1530 Valley Center Parkway
Bethlehem, PA 18017
T (800) 221-8325
F (215) 861-4811

Kluber Tribologyr
54 Wentworth Ave.
Londonderry, NH 03053
T (603) 434-7704
F (603) 434-8046

William Popoli
President
IBAG North America
Division of Burmco, Inc.
80 Republic Drive
North Haven, CT 06473
T (203) 407-0397
F (203) 407-0516

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