Home- Blogs- Bearing Knowledge-Backlash and Clearance in Crossed Roller Bearings
Backlash and clearance are often used interchangeably when engineers discuss crossed roller bearings, yet they describe different conditions. Clearance is the internal gap between the rolling elements and raceways before external load is applied. Backlash is the lost motion observed at the output when direction reverses, and it includes clearance plus deflection from supports, gears, and housings. For designers of rotary tables, robotics, medical equipment, and precision stages, confusing the two can lead to poor stiffness, positioning errors, or excessive torque. This article explains how clearance and backlash behave in crossed roller bearings, how they influence performance, and how to specify the right condition for a given application. BIBO Bearing provides crossed roller bearing solutions for engineers who need reliable motion control.
Clearance is a dimensional property of the bearing itself. In a crossed roller bearing, cylindrical rollers are arranged alternately at 90 degrees between two raceways. This design supports radial, axial, and moment loads simultaneously, which makes it popular for rotary tables, index tables, and robotic joints. The internal clearance is the total possible movement of one ring relative to the other in the unloaded state. It can be measured in the axial direction, radial direction, or as angular play depending on the bearing configuration and the manufacturer's specification.
Clearance can be positive, zero, or negative. Positive clearance means there is a gap between rollers and raceways. Zero clearance means the surfaces are in contact with no measurable free play and no significant preload. Negative clearance, often called preload, means the rollers are pressed against the raceways before external load is applied. Preload removes internal free play but also increases friction and heat. The right choice depends on the balance between stiffness, torque, speed, and life.
Internal clearance is usually verified before installation or in a controlled fixture. A dial indicator is placed against one ring while the other ring is held. A small reversing axial or radial force is applied, and the displacement is recorded. The measurement must be taken at a stable temperature because thermal expansion changes clearance. For crossed roller bearings, axial clearance is often the most practical indicator because the bearing is frequently mounted with an axial clamping arrangement. If the bearing is preloaded, the measured value may be negative or may require a specified torque to rotate. Because clearance affects installation, it should be confirmed with the bearing supplier before final assembly.
Backlash is the lost motion that occurs when the direction of motion reverses. It is a system-level property, not just a bearing property. In a crossed roller bearing assembly, backlash can come from internal clearance, elastic deformation of the rings, mounting fits, housing stiffness, drive couplings, gear reducers, and even the control loop of a servo system. For a rotary axis, backlash is often measured as angular error at a known radius. For a linear axis, it is measured as linear displacement. The key point is that zero clearance in the bearing does not automatically mean zero backlash in the machine.
Internal clearance creates a free movement zone. Under a reversing load, the rolling elements travel across that zone before they begin to carry load in the opposite direction. Preload reduces or eliminates this free movement zone, so the bearing responds immediately to a change in direction. However, preload cannot compensate for soft mounting plates, loose fits, or a flexible machine frame. If the housing deflects under load, the output can still lag behind the command. Therefore, engineers should specify bearing clearance together with mounting tolerance, surface flatness, and structural stiffness. Temperature is another factor: as the bearing warms up, clearance may increase or decrease depending on the materials and mounting arrangement.
Clearance and backlash influence nearly every performance metric of a crossed roller bearing system. The following effects are the most important during selection and troubleshooting.
Selection starts with the required motion performance, not with a catalog number. Engineers should define allowable backlash, required stiffness, maximum torque, operating speed, temperature range, and mounting accuracy. In crossed roller bearings, the choice is usually between a clearance version, a zero-clearance version, and a preloaded version.
BIBO Bearing supports this decision by matching bearing clearance and preload to the customer's mounting, load, and accuracy requirements. A bearing that is correct on paper can still perform poorly if the housing is not flat, the clamping force is uneven, or the shaft and housing fits are wrong. For this reason, selection should always include the full assembly, not only the bearing.
Backlash and clearance are only useful if they can be measured and controlled. During installation, mounting surfaces must be clean, flat, and free of burrs. Tightening should follow a consistent sequence to avoid distorting the rings. After assembly, backlash can be checked by locking one part of the axis, applying a small reversing force, and measuring movement at the output with a dial indicator or laser interferometer. For a rotary axis, the measurement radius must be recorded so angular backlash can be calculated. For a linear axis, the indicator should be placed as close as possible to the load point.
Clearance can be adjusted in some crossed roller bearing designs by grinding spacers, changing clamp force, or selecting a different clearance group. However, adjustment should not be attempted without understanding the effect on preload and life. Over-tightening a bearing to remove backlash can create brinelling, high torque, and rapid failure. If the application requires a specific backlash value, the measurement should be repeated at operating temperature or after a warm-up cycle. BIBO Bearing recommends documenting the measurement method, fixture, load, and temperature so results can be compared across builds.
No. Clearance is the internal gap inside the bearing before external load. Backlash is the total lost motion in the system when direction reverses. Backlash includes clearance plus deflection and play from other components.
Yes. Crossed roller bearings can be supplied with zero clearance or preload. Zero clearance means there is no measurable internal free play, while preload means the rollers are loaded against the raceways before external load is applied.
Preload removes bearing-related free play, but it cannot remove backlash from gears, couplings, weak housings, or control systems. A stiff bearing in a flexible structure can still produce significant lost motion.
Start with allowable backlash, required stiffness, torque limit, speed, temperature, and mounting accuracy. High-accuracy reversing axes usually need preload or zero clearance. High-speed or high-temperature axes may need a controlled clearance to avoid thermal preload.
Some designs allow adjustment through spacers, clamping force, or a different clearance group. The adjustment must be measured and must not exceed the bearing's preload limits. Consult the bearing manufacturer before modifying the assembly.
Clearance and backlash are related but not interchangeable. Clearance is a bearing specification; backlash is the result of the complete mechanical system. In crossed roller bearings, the right clearance or preload improves stiffness, positioning accuracy, and load distribution, while the wrong value increases friction, heat, vibration, or lost motion. Engineers should define the motion requirements first, then select the bearing and mounting method together. BIBO Bearing works with design teams to evaluate clearance, preload, and installation so that the final axis achieves the required precision and service life.
