Follower-Roller: Design Specifications

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Follower-Roller: Design Specifications

Follower-Rollers: Design Specifications

Commercial Follower-Rollers are optimized for rolling along a Cam-Profile, or a Cam-Track. It is helpful to understand their design features.

Compare a Housed Bearing and a Follower-Roller Bearing

Contact Stress evenly distributed around outer ring when in a 'Bearing Housing'.

Contact Stress/Pressure of Bearing in Housing

Housed Bearings : The outer-ring is housed (in a housing, or a 'house')

Typically, the outer-ring is fixed in a housing and the inner-ring rotates with a shaft.

The contact between the outer ring and its housing is 'conformal' and with a small interference fit to make sure the bearing ring does not rotate in the housing. The load is distributed around the outer housing.

Thus, the outer-ring does not distort when a force is applied, since it is supported by the housing (unless the housing distorts).

<< The image to the left shows the bearing supported in a housing. The load around the outside of the bearing is a function of the fit between the housing and the bearing - the pre-load.

Contact Stress concentrated at contact when outer bearing ring in contact with a cam or track.

Contact Stress of Bearing against Cam

Follower-Roller Bearings : The outer-ring is not housed (not in a 'house')

The outer-ring rotates and the inner-ring is fixed.

The contact between the outer-ring and the Cam-Profile is a line (or a very small patch) across the width of the Cam-Profile and Follower-Roller. The Hertzian contact stress is relatively high.

There is a tendency for the outer-ring to distort when a force is applied. To reduce distortion, the cross-section of the outer-ring is deeper than that of a normal bearing.

<<< The image to the left shows a Follower-Roller against a Cam-Profile. The red spots indicate symbolically the contact-stress.

Maximum Follower-Roller Angular Acceleration

Maximum Cam Follower Acceleration

The outer-ring of the Follower-Roller does not rotate at constant angular velocity.

Nominally, the outer-ring's :

Minimum angular velocity is when it is in contact with the cam's Base-Circle

Maximum angular velocity is when it is in contact with the cam's maximum radius (its 'nose' in the case of the sketch to the left).

The Follower-Profile must accelerate and decelerate as the radius of the cam changes.

However, the maximum possible angular acceleration of the Follower-Roller is a function of its radius, inertia, and friction between it and the cam-profile. The friction-force is not constant because the contact-force is not constant.

- maximum follower acceleration

- friction force

- outside diameter of follower

- inertia of roller

Maximum Follower-Roller Rotational Speed

Nominally, the maximum Angular Velocity of the Cam-Follower Bearing is at the maximum radius of the Cam

Nominally, the maximum Angular Velocity of the Cam-Follower Bearing is at the maximum radius of the Cam

Maximum Rotational Speed (RPM).

The maximum possible speed of a bearing is a function of its design and how it is lubricated. Its maximum speed is usually given in the catalog. Alternatively, you can use this table (below) and this simple equation:

Table Parameter = Stud Dia. (or mean diameter)(d) × Max. Speed(RPM)

E.g.: The maximum rotational speed of a Follower-Roller bearing; with an internal diameter (stud diameter) of 10mm;

Grease Lubrication:

Oil Lubrication:

Bearing Configuration :

Grease

Oil

Needle Rollers with Cage

84 000

140 000

Needle Rollers as Full compliment

42 000

70 000

Cylindrical Stud Rollers

Track-Followers

66 000 / 72 000

110 000 / 120 000

Minimum Load

A pre-load can increase stiffness, reduce noise, improve shaft guidance, extend bearing life, improve running accuracy, prevent skidding of the internal needles, rollers, or balls relative to the inner and outer rings.

The minimum radial load is approximately:

Skew and Tilt

SKEW [view of Roller above Track]

SKEW
[view of Roller above Track]

TILT  [sectional view of Roller and Track]

TILT
[sectional view of Roller and Track]

Cylindrical Follower-Rollers

SKEW

Rollers that skew increase the axial load and axial slippage between the Cam-Profile and Follower-Roller. The Follower-Roller skew angle specification is typically:

Skew Angle < 1.4 x 10-4 × Contact-Stress (°) or < 2.5 x 10-3 × Contact-Stress m.rad.


TILT

The maximum tilt capability of:

Cylindrical Roller (typical) < 0.1°(1.7m.rad).

Barrel Roller (typical) < 0.25° (4.4m.rad)

Barrel Follower-Roller

The outside surface of a Barrel Follower-Rollers is not cylindrical. They have a surface that has a radius They are intended to be used when the rotational axis of the Follower-Roller cannot be guaranteed to be parallel to the rotational axis of the cam.

Barrel Follower-Rollers are also called Crowned Follower-Rollers, or even Spherical Follower-Rollers. Others have a surface described as Logarithmic Follower-Rollers - their actual profiles are commercially sensitive.

Maximum Contact Stress of Cyclindrical and a Barrel Cam-Followers

Schematic of Maximum Contact Stress of Cylindrical and Barrel Cam-Followers
Schematic borrowed from INA bearings catalog.

A Cylindrical Follower or a Barrel Follower?

When to use a Cylindrical-Follower-Roller or a Barrel Follower-Roller is a function of the type of machine and how well you can align the Follower-Roller and Cam-Shaft rotational-axes.

Cylindrical Roller - and misalignment

When the rotational axis of a Cylindrical-Follower-Roller is not parallel to the rotational-axis of a Cam-Shaft, the Follower-Roller tilts relative the cam's axis. A Cylindrical Follower-Roller that tilts, will roll along its edge - see the bottom and left example in the image above.

The contact is distorted. The maximum value of Contact-Stress is significantly greater than the nominal value that is calculated for 'line-contact'.

Barrel Rollers.

If the rotational-axis of the Follower-Roller is not parallel to that of the cam, the Follower-Roller will tilt relative to the cam's surface.

In this case, the contact moves across the cam surface, but when the tilt-angle is within limits, it does not roll along its edge.

The contact is not distorted much. The maximum value of Contact-Stress is not more than the nominal value that is calculated for elliptical-contact.

The calculated contact-stress of a Barrel Follower-Roller is greater than that calculated for a Cylindrical Follower-Roller.

However, when a Follower-Roller tilts the actual contact-stress experienced by a Cylindrical Follower-Roller is much greater than that calculated for a Barrel Follower-Roller.

The permissible tilt angle of a Cylindrical Follower-Roller is very small,  <0.1°, and thus a Barrel Follower-Roller is a good design option in many cases.

If the Follower-Roller does-not tilt then the:

maximum contact stress of a cylindrical Follower-Roller is less (marginally) than that of a barrel Follower-Roller.

If the Follower-Roller does tilt then the:

maximum contact-stress of a cylindrical Follower-Roller is more than the that of a barrel Follower-Roller.

Materials and Size Tolerances

Outside Diameter Tolerance of Bearing

Nominal outside diameter of a cylindrical Follower-Roller Bearing is: ~h5 , nominally zero to a small under-size.

Nominal outside diameter of a crowned Follower-Roller Bearing is: 0.00 to –0.050mm , nominally zero to under-size.

Typical Yoke Cam-Follower Design

Typical Yoke Cam-Follower Design

With INNER-RING

Surface Finish (Shaft/Pin)

Maximum roughness = Ra =0.4μm

Diameter Tolerance

Shaft/Pin : g6

Form Tolerances

Variation : Mean Shaft Diameter = 0.25 × Diameter Tolerance

Variation : Circular Form = 0.5 × Diameter Tolerance

GDI-Cam-Follower-Yoke-Arrangement-No-Inner-Housing

Without INNER-RING

When space is limited, it is possible to use a Follower-Roller that does not have an inner-ring.

The Shaft/Pin has a specification that closely replicates the inner-ring of the bearing.

Surface Finish (Shaft/Pin)

Maximum roughness = Ra =0.4μm

Diameter Tolerance

Shaft/Pin : k5

Shaft / Pin Steels:

Through-Hardened Steel :

ISO 683-17 : 100Cr6

Case-Hardening Steel :

ISO 683-17 : 17MnCr5, 18CrNiMo7-6 or

EN 10084 : 16MnCr5, Ck15, 15Cr3, 17Cr3

Flame and Induction-Hardening Steel :

DIN EN ISO 683-17 : 43CrMo4, or

DIN 17212 : Cf53.

Hardness

700HV +170 HV

or:

58 to 64 HRC (Rockwell Hardness Scale 'C')

If the hardness of the pin/shaft is less than 58HRC, the load-capacity of the Follower-Roller is reduced. This table shows how dynamic and static load capacities become reduced as the shaft's hardness is reduced.

Hardness

HRC

60

58

56

54

52

50

48

45

40

35

30

25

HV

697

653

613

577

545

512

485

447

392

346

302

267

Reduction

Factor

Dyn.

1.0

1.0

.93

.84

.73

.63

.52

.43

.31

.23

.15

.11

Static

1.0

1.0

1.0

1.0

.96

.86

.77

.65

.50

.39

.30

.25

Without INNER-RING... continued

Hardening Depth

Minimum Case Hardening Depth, CHD, at which hardness is greater than 550 HV, HRC 52.3:

Minimum Surface Hardening Depth, (from Flame and Induction Hardening) SHD

or, always:

Depth of Hardness > 0.5mm.

Surface Finish:

For high speed and loads: wave free finish Ra= 0.2μm . Includes shoulders and washers, if in contact with the rollers ends, or bearing rings.

General Applications: wave free finish Ra=0.35μm

Tolerance and Form: k5, k6

Variation of Mean Diameter: <0.008mm, or 0.5×diameter tolerance, or <5μm/Φ25mm, whichever is the least

Deviation from Circular Form: <0.0025mm or 0.25×diameter tolerance, or <2.5μm/Φ<25mm, whichever is the least

High Frequency Lobing: Lobing is 10 or more times around the circumference of a shaft. If it exceeds 0.4μm from peak to valley it is called chatter. Chatter usually causes undesirable noise and reduces fatigue life.

Shaft Slope: Slant precision <13μm/25mm.

General

No nicks, burrs, scratches and dents. Oil holes are permissible in the raceway area, but care must be taken to blend the edges gently, in to the raceway, if possible put oil-hole in the unloaded zone. No grind reliefs, fillets etc

Shaft Material:

 

JIS (Ja)

ASTM (USA)

DIN (DE)

Through-Hardening Steel

SUJ2

AISI 52100

100Cr6

SK3 (SK2)

 

 

Case-Hardening Steel

SNCM420

SNCM 8

4320

4340 (1.6565)

16CrNiMo6

17MnCr5, 16MnCr5

15CrNi6

SCr420

5120

20Cr

SCM420

4118

25CrMo4

SNC420(SNC415)

 

14NiCr10

Follower-Rollers - Stud type

GDI-Cam-Follower-Stud-Arrangement-CantileverForces

The stud extends to the side of the bearing.

Support Flange and Dimension, F

To support the side plate of the bearing, the shoulder should be ground flat, and be larger than dimension F. Dimension F is given in catalogs.

The material of the Follower must support the compressive stresses of the tightening torque - see more below.

Stud Diameter

A metric stud has a 'shaft' tolerance of h7. This is a zero to negative tolerance.

An imperial (inch) stud is oversized by approximately 0.025mm.

Hole through Follower

Metric Hole tolerance : JS7 (J7). This is a small plus and minus tolerance of the nominal hole size.

Inch Hole tolerance : F7. This is a positive tolerance on the hole.

In both cases, the stud should have a light press fit.

Stud Stress

The stress on the stud is a function of two parameters:

Tightening Torque : produces a Tensile Stress, σt.

When the guidelines are followed, the tensile stress, σt , is approximately 100MPa.

Contact Load, P, produces a Bending Stress, σb

The bending stress, is:

σb = P.L.y/J

σb = P.L./z  

Note: J, the second moment of area, is reduced when there is an oil-hole through the center of the stud.

 

Stud fails if:

Total Stress = Bending Stress, σb + Tightening Stress, σt Tensile Strength, or Yield Stress, σyield

σt + σb > σyield  

Rules of thumb: the total stress on the stud should be less than:

when load is static

when load is dynamic but uni-directional

when load is bi-directional and dynamic

Thread fails if:...

...the nut becomes loose. This occurs when:

Compression stress (from bending) is greater than the Tension stress due to the nut's tightening, the nut might become loose.

Follow the nut tightening torque, below!


Nut Tightening Torques (EN ISO 4032).

M6x1(3Nm) ; M8x1.25(8Nm) ; M10x1(15Nm) ; M12x1.5(22Nm) ; M16x1.5(58Nm) ; M18x1.5(87Nm) ; M20x1.5(120Nm) ; M24x1.5(220Nm) ; M30x1.5(450Nm)

Follower-Rollers - lubrication

Oil lubrication

For oil lubrication, oils of type CLP to DIN 51517 are recommended.

Grease lubrication

For grease lubrication, lithium soap greases to DIN 51825 should be used. Re-lubrication intervals can only be determined under operating conditions. Re-lubrication must be carried out at the latest when fretting corrosion first occurs; this can be identified by a reddish discoloration of the mating track or the outer ring.

Solid lubricants and antifriction coatings

These substances are also suitable for lubrication. At high traverse or rotational speeds, however, they provide effective lubrication for a significantly shorter period than oil or grease lubrication.

Dirt Ingress

To prevent dirt ingress: Install the oil plug if you are not going to re-lube it.


Notes:

To prevent the stud rotating: Insert a pin into the lubrication hole (if present in stud).

Ideally, locate the lubrication hole (hat is across the shaft)  in the unloaded direction.

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