Radial Play, Axial Play and Contact Angle

All of the above are interlinked with the internal geometry of a ball bearing, If you change one of the above, then the others will also change. To ensure the optimum performance from a ball bearing, it is often more important to consider the internal geometry, especially if the bearing is going to be subjected to extreme running conditions. Great care must be taken when applying interference fits or where dis-similar materials are used (with different coefficients of expansion) over wide temperature ranges.

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What is Radial play 'RL'?

The radial play is the total radial displacement of the outer ring against the inner ring. Radial play has nothing to do with quality of a bearing but can be a major factor when considering bearing performance & life.

Radial Play = Outer Raceway Diameter ' (Inner Raceway Diameter + 2 x Ball Diameter)

A higher radial play will:

• To a degree compensate for interference fits.

• Offset some expansion and contraction in shafts and housings due to temperature change.

• Provide a greater contact angle to allow for greater axial loads.

• Increase axial stiffness in a pre-loaded pair.

• Allow a small amount of mis-alignment.

A tighter radial play will:

• Limit the deviation of the balls within the running track to a truer circle which reduces skidding and reduces wear.

• Reduce the axial play.

• Reduce the tilt angle, vibration and noise.

What is Axial Play 'AL'?

The axial play is the total axial displacement of the outer ring against the inner ring.

Where:

Da = profile diameter outer ring

Di = profile diameter inner ring

dk = ball diameter

RL = radial play

What is the Contact Angle?

The contact angle is the angle between a vertical line through the centre of the ball perpendicular to the axis of rotation and a straight line crossing the contact points of the ball at the raceways.

Where:

αo = Contact Angle

da = profile diameter outer ring

di = profile diameter inner ring

dk = Ball Diameter

RL = Radial Play

What is Tilt Angle?

The angular displacement of the inner ring axis in relation to the outer ring axis it should be kept to a minimum to ensure good contact of balls to the raceways.

What is the Running Track?

The path the balls follow around the raceway; this should be as near to a perfect circle as possible to reduce the skidding and associated wear created by the balls speeding up and slowing down during each rotation as would be the case if the inner ring were tilted causing an elliptical running track.

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What is Raceway Curvature?

The raceway curvature (x) is the proportional difference between the diameter of the ball and the diameter of the raceway profile, it is usually stated as a percentage of the ball diameter.

Where:

x = Curvature

de = Raceway Profile Diameter

dk = Ball Diameter

Example:

If the chosen figure Curvature (x) = 1.14, for a given ball and a specified Radial Play (RL):

If x < 1.14 (curvature is tighter) then:

• Contact Angle (α

  o

  ) is larger

• Axial Play (AL) is smaller

• Dynamic Load Capacity (C) is larger

• Static Load Capacity (C

  o

  ) is larger

If x > 1.14 (curvature is shallower) then: 

• Contact Angle (α

  o

  ) is smaller

• Axial Play (AL) is larger

• Dynamic Load Capacity (C) is smaller

• Static Load Capacity (C

  o

  ) is smaller

Please note this guide provides theoretical generalisations only. Please contact us regarding specific applications for more specific, technical advice and we will be happy to assist.

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Definition and Importance

The contact angle in bearings is a critical parameter that defines the angle between the direction of load on the bearing and the nominal line of action of the resultant forces transmitted by a bearing raceway member to a rolling element. This angle plays a significant role in determining the bearing's load capacity, friction, rigidity, and ability to withstand mounting errors.

ISO Standard

According to ISO standard for linear motion rolling bearings, the nominal contact angle is defined as:

The angle between the direction of load on the linear bearing and the nominal line of action of the resultant of the forces transmitted by a bearing raceway member to a rolling element.

Types of Contact Angles

1. 45 Degree Contact Angles for Equal Load Capacity

   • Bearings with Gothic arch geometry, including miniature profiled rails and most roller bearing guides, typically have four points of contact between the ball and raceway. This results in a contact angle of 45 degrees.
   • The benefit of this 45-degree angle is that it provides equal load capacity in all four primary directions: radial (downward and lift-off) and lateral (side) loading. This means that such guides can be used in any orientation without needing to de-rate their load capacity.

2. Higher Contact Angles for Better Radial Load Capacity

   • Bearings using circular arc or offset Gothic arch geometry can have varying contact angles designed to produce higher load ratings in one direction at the expense of other directions.
   • For instance, one design uses a 90-degree contact angle on top rows of balls with a smaller 30-degree angle on lower rows. This configuration offers extremely high load capacity for radial (downward) loads and high rigidity when radial loads are applied but sacrifices load capacity and rigidity in reverse radial and lateral directions.
   • Another design based on offset Gothic arch geometry employs a 50-degree contact angle for all four rows of balls, providing higher load capacities in both radial and reverse radial directions but lower lateral load capacity.

Impact on Bearing Performance

• Load Capacity: The contact angle directly influences how much load a bearing can support. A higher contact angle generally increases radial load capacity but may reduce axial or lateral capacities.
• Rigidity: Bearings with specific contact angles can offer greater rigidity (lower deflection) under certain loads, which is crucial for applications requiring precise movement.
• Friction: The geometry affecting the contact angle also impacts friction levels within the bearing system.
• Mounting Errors: Properly chosen contact angles help bearings withstand mounting inaccuracies without significant performance degradation.

Understanding these aspects allows engineers to select bearings that best meet their application's demands regarding load distribution, rigidity, friction management, and tolerance to mounting errors.

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