Ballscrew Thrust Bearings

7/29/2019 Ballscrew Thrust Bearings

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Machine Tool Systems, Inc.

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Ball Screw Thrust Bearings.pub

Angular Contact Thrust Bearings ( TAC-Series )

Angular –Contact ball screw thrust bearings are usually used as preloaded combinationsThe “standard” preload designation in a bearing combination is C10 for metric series.However, Okuma does have thrust bearings specified with special preloads for their application. Care should be taken when trying to substitute or obtain TAC bearings fromlocal distributors.

Construction

The faces of ball screw thrust bearings are precisely ground to achieve a specified pre-load when the set is axially assembled and tightened. The bore and outside diameters withiaset are selectively matched to much less that tolerance class PN7A so that bearings loadsare distributed evenly.

1. Thrusting

All TAC bearings as used for ball screwshave two different faces known as the FACE

and BACK . As shown in Fig. 2-2, the face side of the bearing inner and outer races areoffset axially where the inner race is extendedcompared to the outer race. Also note thatthe back can be identified by the way theinner race is recessed compared to the outer race. Therefore, the bearing has a diffidentthrust direction as indicated by the thrust linespassing through the balls form the face of theinner race to the back of the outer race.

2. Preload

As mentioned above, preload is obtained by precisely grinding the faces of each racefor a specific amount. For example, a 35TAC72B C10 bearing will have 3800 NewtonMeters of preload.

Fig. 2-1

Stick Out

DU

Stick Out

Force

Force

Stick Out of Inner & Outer

Races

Face

Back

Preload

Thrust

Fig. 2-2




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Composition of Bearing Numbers

Class

The Standard NSK bearing tolerance class is PN7A. It conforms to ISO Class 4 for radial bearings except that a more strigent standard is used for the axial runout of the inner

and outer races. In case of universal-combination bearings (SU type), Class PN7B is used.

No symbol: ISO class 0........ ( ABEC 1)P6: ISO class 6........ ( ABEC 3)P5: ISO class 5........ ( ABEC 5)P4: ISO class 4........ ( ABEC 7) …….PN7A & PN7B

F a

F a

Preload

NSK 30TAC62DBPN7A

DB Arangement

60 Degree Angular

Contact

DB Arangement

F a

F a

Preload

NTN BNT7206T2DB/GNP4

30 Degree Angular

Contact

NSK 30 TAC 62 DB C10 PN7A

Accuracy SymbolPreload SymbolCombination SymbolOutside Diameter (mm)Bearing Type (Contact Angle, 60°)Bearing Bore Diameter (mm)

NTN BNT 7 2 06 * T2 DB / GN P4

Tolerance: ISO class 4Preload: Normal Preload

Arrangement: Duplex pair, back to back mountingCage: Plastic cage, nyolin or teflon

Contact Angle: 30° (blank)Bore Diameter: 30 mmSize:Series: Angular contact ball bearingPrefix: High Speed angular contact




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Preload

Duplex thrust bearings use a matched set of two bearings clamped solidly together onthe ball screw shaft. In order to achieve proper preload the thrust bearing lock nut must betightened with sufficient torque to cause the faces of the two inner races to come together.In that way specified preload is achieved.

Preload

Duplex Back-to-Back

25TAC42DB

F a

F a

Lock Nut

Ball Screw

Zero Clearance

Zero Clearance

TAC Bearing Assembly“ Loaded “

Fig. 2-3

Fig. 2-4




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Bearing Combinations

Figures 2-5 and 2-6 show the various bearing combinations and their thrustingdesignations.

The most common configurations Okuma uses for ball screws is the back-to-back DB.However, Tandem DT, Triplex DFD and DBD arrangements are also used. as show in

Fig. 2-5 and 2-6. It is important to observe the correct mounting of the set. Bearingsmarked as DB will only thrust properly when arranged back-to-back. As shown here, a“V” is marked on the periphery of each set to insure correct installation.

Face- to- Face

arrangement

DF

Tandem

arrangement

DT

Back- to- Back

arrangement

DB

Duplex Arrangements

Fig. 2-5

DFDDTD

DBD

Triplex Arrangements

Fig. 2-6




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Other Combinations

Universal ground types are designated as SU or DU sets, can be assembled in any of thbasic configurations ( DF, DB, DT ), or used alone. Tolerance Class PN7B is specified toreduce the bore and outside diameter variations within any resulting set. A “V” mark issometimes scribed to indicate proper preloading direction.

Okuma does not recommend using DU or SU sets due to required bearing preloads for any given axis, unless they are supplied by them. If standard “SU” sets are used, resultinaxis backlash, (lost Motion) may be greater than normal. If a DU or SU set is used, proper configuration must still be observed, otherwise positioning accuracy, lost motion, and per-formance will be greatly affected.

“SU” set: Single bearing in one box. May be installed with any other SU bearing from

another box in any combination or direction. LESS ACCURACY.

“DU” set: Two matched bearings in one box. May be installed in any combination or

direction. BETTER ACCURACY.

DU / SU

Back - to - Back

Preload

ForceForce

Force For

Preload

Face - to - Face

DU / SU

DB Configuration DF Configuration

Universal Sets / Flush Ground Type

Fig. 2-7

Fig. 2-8




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TAC Bearing Installation

Correct mounting of thrust bearings is essential to proper operation. Care must be takento ensure that the set is put together in the right configuration. Most all precision thrustbearing sets have a scribed “V” mark to indicate the correct arrangement. Figure 2-9 showsa 2-bearing set in a DB configuration. This set can only be assemble in this manner tofunction properly, otherwise they will not load up correctly.

Also, the “V” mark indicates the proper DIRECTION of mounting. With duplex sets the small end of the“V” should always points toward the BALL SCREW NUT . The lock nut should be against at the largeend.

NOTE:

Triplex sets are also used on tailstock live quill shafts. Fig. 2-10

DB

“V” Mark

Fig. 2-9

Ball Nut

“V” Mark

Figure 2-10 shows a triplex, (3-Bearing) set in the DFDconfiguration. Again, observe the proper mountingarrangement. The “V” mark may or may not point towardthe ball screw nut depending upon the axis and machineinvolved. Please observe how the old bearings comeout during disassembly.

DFD




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Run-Out Marks

Precision thrust bearings also have “o” marks scribed on the faces of the inner andouter races as shown in Fig. 2-11. These circle marks identify the position of maximumrun-out in the bearing and great care must be taken during mounting to install the bearingscorrectly

Installing Bearings with Circle Marks

Why are circle marks important?

When bearings are assembled, the run-out marks are used to minimize the effective runout in the set. If this is not done correctly, bearing life, accuracy, and performance will begreatly affected. Install the bearings on the shaft with circle marks on the inner race in-linewith the shaft keyway.

Effect:

When two bearings are assembled where the points of maximum run-out on the inner races are aligned in a radialmanner the total effective run-out will be at it’s lowest amount.If the marks are not aligned properly, then the two bearings willoppose each other causing “Wobble” which will lead to prematurewear.

“0”

Wrong

Outer race point of maximum run-out

Inner race point of maximum run-out

Lock Nut Keyway

Fig. 2-11

Fig. 2-12




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Checking TAC Bearing Preload

1. Preload of the TAC bearing set must be checked prior to assembly. This is done by firstmeasuring the depth of the housing bearing bore, Dimension “A”, Fig. 2-13

2. Next, place the bearing set on the end cap and measure dimension “B”, Fig. 2-14

The proper difference between dimensions “A” and “B” iswhere “B” is longer than “A” by .100 mm ~ .127 mm.

( .004” ~ .005” )

3. If dimension “B” is more than .2 mm ( .008” ) longer

than “A”, then the face of the cap must be ground.

4. If dimension “B” is equal to “A”, or is shorter, then theshoulder must be turned.

Note: The shoulder and Face mustbe parallel within .005 mm ( .0002” )

Measure“A”

Fig. 2-13Measure

“B”

Fig. 2-14

B = ( A +.100 ~ .127 mm )

Fig. 2-15

Grind the cap faceif too long

Turn the shoulder if too short




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Housing

The bearing housing must be installed on the ball screw shaft prior to installing theTAC bearings. Ensure that the housing is clean and stone the face. Also if there is aball screw bellows or spring steel cover be sure to install it first.

Slip the Housing over the ball screw being careful not to damage the seal when sliding

over the ball groves. The housing face should be past the bearing shoulder.

Housing

Fig. 2-16

Stone

Seal




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Bearing Preperation

Handling and Installation

Improper handling prior to and during installation can easily damage the performance

and life of the bearings. It is necessary that certain precautions are taken to prevent suchdamage.

The most important factor to keep in mind when handling bearings is cleanliness. Everyeffort should be taken to keep them clean.

Lubrication

TAC bearings come carefully packaged with special grease or oil to prevent oxidation.This packing grease will have to be washed from the bearing very carefully. Use a solventsuitable for cleaning bearings which is very clean. However, cleaning the protective lubricant

from new bearings should only be done at the last possible moment. Packing each bearingwith the proper grease should be done immediately after cleaning. If they are not to beimmediately installed, then they must be placed in a clean plastic bag until such time as theyare mounted.

Bearings should be cleaned, packed, and installed in as close to a clean roomenvironment as possible. Dirty conditions in the bearing installation area can easily lead tocontamination and premature failure.

Recommended Lubricant

NSK and NTN bearings should be greased with a lithium soap based grease such asNSK No. 1 or No. 2 grease. These are available from your NSK bearing distributor.However, any high quality EP-No. 2 bearing grease will work if it is lithium soap based.

How Much?

The saying “More Grease Is Better” can’t be further from the truth.Excess grease packing can be as detrimental as not enough. Excessivegrease causes increased friction which results in rapid heat generation.This may lead to unexpected failure when operated at high speeds.Since ballscrews rotate at relatively low speeds, there is not a specific

amount of grease to pack into each bearing. However a good rule of thumb is to fill each bearing approximately 75 ~ 80% full.

The use of protective Nitrail gloves is recommended so not tocontaminate the grease with the hands.

Fig. 2-17




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Bearing Installation

After the bearings have been packed with grease, and the shaft has been thoroughlycleaned and inspected for burrs, they are now ready to be mounted to the shaft. Asmentioned, the points of maximum run-out and “V” marks must be aligned with theball screw properly. The lock nut keyway may be used as a reference guide to insure

proper installation as shown in Fig. 2-18.

Proper assembly procedures and tools such as bearing sleeves will avoid damage tothe bearings during mounting. When a a bearing is pressed on a shaft, the inner racemust be started squarely. A “cocked” race may score the shaft and damage the bearing.The bearing must not be tapped in place with direct blows on the bearing race.

The preferred practice of mounting the bearings is to use a sleeve to tap the bearingson the shaft as seen in figure 2-19. The bearings may also be heated to expand theinner race and pushed up onto the shaft. However, this must be done very quickly or therace will shrink on the shaft before the bearings are seated. If that happens, finish bytapping with a sleeve as illustrated.

Keyway Reference

Insure shaft is clean andfree of burrs.

Fig. 2-18

Bearing Mounting Sleeve

Fig. 2-19

Tap Lightly




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Installing Lock Nut & Housing

1. Install the spacer collar and lock nut. If the lock nut binds, tap lightly around theoutside diameter of the nut to re-form the threads.

2. Fully tighten the nut. The outer races should be trusted at this time.

3. Slowly rotate the bearing set to feel for tight spots, ( “ratcheting” ). If tight spots arefelt then the bearings are not seated on the shoulder properly, or the nut is too tight.

4. Loosen the lock nut and snugit back up. Do not fully tightenat this time.

5. Rotate the outer races wherethe “V” lines up with the keyway.

6. Slide the bearing housing upover the bearings until theface is just flush with the frontof the bearings.

7. Torque the lock nut until it is tight.

8. Install the copper set pieces, and set screws and tighten fully.

9. Feel for tight spots in the bearings as before. Bearings should feel stiff but smooth.Bearing / Shaft run-out will have to be adjusted later.

Lock Nut

Collar

Thick Face

Fig. 2-20

Set Screw

Copper Set Piece

Housing

Fig. 2-21




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Installing Bearing Lock Nut Washers

Older lathes use precision lock washers to secure the lock nut instead of set screws.The inner tab must be bent to fit the shaft keyway prior to installing on the shaft. Takecare not to tare the edges when bending.

1. First install and tighten the lock nut without the washer.

This will seat the bearings.

2. Remove the lock nut.

3. Before installing the lock nut, apply a small amountof grease to the face of the lock nut. This will help preventdamage to the lock washer when the nut is torqued.

4. Install the lock washer and lock nut and torque the nut with a spanner wrench.

5. After tightening the lock nut, the lock washer must

be bent down into the slot on the lock nut. The preferredmethod of bending is illustrated in figures 22, and 23.

6. First start the bend with a screw driver as shown in figure2-22.

7. A dowel pin is then used to complete the bend with a largepair of Channel Lock pliers. This will make a very smoothbend and avoid tearing the washer.

Start Bend WithScrew Driver

Never bend the washer down using a punch and hammer!! !

CAUTION

Bearing Lock Washer

Fig. 2-22

A N 0 8

Dowel Pin

Fig. 2-23

Smooth Bend




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Installing Housing End Cap

The end cap was checked and adjusted as necessary prior to this step, however the TAC bearing preload should be re-checked at this time.

1. Install end cap with mounting bolts just snug.

2. With feeler gauges, check preload clearance between the face of the cap and face of the housing. There should be .100 mm ~ .127 mm ( .004” ~ .005” ) clearance.If clearance is not correct, then the end cap will have to be adjusted by grinding or turning it’s faces as before.

3. If preload clearance is OK, then torque all mounting bolts to 75% of their rated torqueas noted in a bolt torque chart for metric class 12.9 bolts.

End Cap

Housing

Preload Clearance

.100 ~ .127mm

( .004 ~ .005" )

Fig. 2-24

Fig. 2-25

Fig. 2-26




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Adjusting TAC Bearing Run-Out

After TAC bearing installation, it is necessary to correct for run-out that is produced whenthe lock nut is tightened. Tightening the lock nut causes uneven load around the face of the bearing race which is due to the thread pitch. Uneven stress on the bearings can beeliminated in the following procedure.

Procedure

Run-out in the TAC bearings is corrected byrotating the ball screw and locating the pointof maximum radial run-out. At this point, thelock nut is lightly taped to relive the stress in

the nut.

1. Set up an indicator to indicate the ballscrew as shown in Fig. 2-27.

2. Slowly rotate the ball screw and find thepoint of maximum run-out.

3. On newer lathes, the end cap has an access hole abovthe lock nut. Remove the pipe plug to gain access.

4. Insert a screw driver thru the hole andlightly tap the nut at the closes slotin-line with the high point as shown in

figures 28, and 29.

5. Continue checking and tapping thenut until the total run-out is below.005 mm (.0004”).

+

0

5 0

5 0

+

Fig. 2-27

Fig. 2-29

Pipe P

Fig. 2-28

Pipe Plug

Do not removeEnd Cap

Tighten

A N 0 8

Place screwdrive blade inslot.

High Point

+

Tap the lock nut in a clockwisedirection only. DO NOT hitstraight down, or the bearingsmay be damaged.

Tap Screw Driver with Hammer.




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Support End Radial Bearings

In some cases, support end radial bearings are also marked with a point of maximumrun-out . Anytime these circle marks are present, proper orientation is important.

Why Care About Orientation ?

Effects of ignoring proper bearing installation can be illustrated in figure 2-27. Wheninstalling the TAC bearings and support end bearings on a ball screw, it is important to aligneach bearing set with their circle marks in-line with each other at each end of the shaft.In that way the total combined run-out can be kept at a minimum. Use the keyways on theball screw as a guide to mount the bearings. Align circle markson the outer race with the top of each housing.

Unlike Angular Contact Thrust bearings, RadialBall bearings do not have a scribed “V” on theouter race.

1. Install each bearing as shown in figure 2-28.

2. After installing the bearings, secure themwith the lock nut and washer. However, donot install the pulley at this time.

Collar

Radial Bearings

Lockwasher

Collar

Locknut

Pulley

Woodruff Key

Mis-Matched

Run-Out Marks

Matched

Run-Out Marks

Mis-Matched

Run-Out Marks

Matched

Run-Out Marks

Excessive Total

Run-Out

Minimal Run-Out

TAC Bearing Radial BearingsBall Nut

Fig. 2-27

6205LLB Circle MarkKeyway

Fig. 2-28

Point of Maximum Run-Outon Inner Race

Fig. 2-29

LC-SeriesUse bearing sleeve toinstall bearings.




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3. After tightening the locknut and securing it with the lock washer, it is necessary toadjust the shaft run-out as shown previously.

4. Continue assembly, check and adjust pulley facerun-out as shown in figure 2-31.

0

5

0 5 0

+

Adjust run-out to within.005 mm (.0002”) T.I.R.

Tap nut at HIGH point.Fig. 2-30

0

50

5 0

+

Fig. 2-31

Adjust run-out within.020 mm (.0008”) T.I.R.

Tap nut at HIGH point.




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Ball Screws With Couplings

With precision drive couplings it is very important to reduce the amount of run-out at the endof the ball screw to as low a value as possible. Maximum allowable is .005mm ( .0002” ) T.I.R.,but try to get it down to .002mm ( .0001” ) T.I.R. if possible.

After adjusting the shaft run-out, install the motor with

coupling as follows.

1. Install the motor and check to see if the motor shaft willrotate without binding.

2. Indicate the coupling ring at the ball screw end and adjustthe total run-out within .010mm ( .0004” ) T.I.R. bytightening each compression ring bolt a little at a time.( Cross torque each bolt 1/4 turn at a time ).

3. Indicate the coupling ring at the motor end and adjustthe run-out as in the same manner.

Run-out must not be more than .010 mm or binding willoccur.

0

5 05 0

+

Fig. 2-32

0

5 0

5 0

+

0

5 0

5 0

+Check Run-Out

Above Each Bolt

Fig. 2-34

IndicateRingsHere

Tighten These Bolts

Motor Coupling

Fig. 2-33

.005mm ( .0002” ) T.I.R.Maximum Run-Out

Ballscrew Thrust Bearings

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