Ball screw support bearings are designed to locate the screw shaft, carry axial thrust and maintain the rigidity required for accurate motion. However, a sudden collision, emergency stop or rapid reversal can generate a short-duration peak load far higher than the axis experiences during normal operation.
Even when the continuous operating load is within the bearing rating, repeated shock loads can damage the bearing raceways, rolling elements, retaining components or shaft-end interfaces. Preventing this type of failure requires more than selecting a larger bearing: the motion profile, mechanical stops, support arrangement and installation accuracy must be considered together.
What Is a Shock Load in a Ball Screw System?
A shock load is a force that rises sharply over a very short period. In a ball screw axis, it may be generated when:
- The moving table reaches the end of its travel
- The carriage collides with an obstruction
- The motor starts, stops or reverses too aggressively
- An emergency stop decelerates a heavy moving mass
- A vertical axis loses control and drops
- Backlash or looseness allows components to strike during reversal
- An external impact is transmitted through the machine structure
These events can produce a much higher instantaneous force than normal cutting, positioning or transport loads. The resulting force passes through the carriage and ball nut into the screw shaft and its support bearings.
Which Components Can Be Damaged?
The phrase "ball screw bearing" is sometimes used loosely, but several different components may be affected by shock.
Fixed-end support bearing
The fixed end normally carries axial load in both directions and establishes the screw's axial position. It is often the support most directly exposed to a sudden thrust load.
Floating-end bearing
The floating end primarily supports the screw radially while allowing the axial movement required by thermal expansion. Incorrect assembly that locks this end axially can cause impact and thermal loads to be shared in an unintended way.
Ball nut and screw raceways
Shock transmitted through the nut can create high contact stress between the balls and raceways. Severe or repeated impact may cause permanent indentations, surface fatigue or changes in running torque.
Coupling and shaft end
A sudden stop can also overload the coupling, bearing locknut, machined journal or mounting bolts. Replacing only the support bearing may therefore fail to correct the complete problem.
How Shock Loads Damage Support Bearings
A bearing may be permanently damaged even if it still rotates after an impact. The most common damage mechanisms include:
- Raceway indentation: concentrated contact stress produces permanent marks where the rolling elements contact the raceway.
- Rolling-element damage: balls may develop surface marks, cracks or local deformation after a severe impact.
- Preload change: deformation or movement in the bearing arrangement may alter the original preload.
- False brinelling or fretting: repeated small oscillations under load may mark the contact surfaces even without full rotation.
- Loss of alignment: an impact may shift the support housing, nut bracket or coupling position.
- Fastener movement: loose support-unit or nut-housing bolts allow additional impact each time the direction changes.
Typical symptoms include new noise, increased vibration, uneven running torque, axial movement, localized heating and reduced positioning repeatability.
1. Calculate the Peak Load, Not Only the Normal Load
Support-bearing selection should not be based only on the steady thrust required to move the machine. Acceleration, deceleration, emergency stopping and possible collision conditions must also be reviewed.
The force required to accelerate or decelerate the moving mass is related to:
F = m × a
where F is the inertial force, m is the moving mass and a is the acceleration or deceleration.
A shorter stopping time produces higher deceleration and therefore higher force. If the table strikes a rigid mechanical stop, the actual peak may be much higher than the force predicted by the normal programmed deceleration.
Compare the calculated operating and peak loads with the permissible loads of the complete system, including:
- Ball screw and ball nut
- Fixed-end support bearing
- Floating-end support
- Coupling
- Nut bracket and support housings
- Mounting bolts and machine structure
Use the manufacturer's allowable static and dynamic load data together with an appropriate safety factor for the actual operating conditions.
2. Use a Controlled Motion Profile
Aggressive acceleration does not always produce an immediate collision, but it can create repeated peak forces during every operating cycle. The control system should avoid unnecessary step changes in velocity or torque.
Depending on the controller, shock can be reduced by adjusting:
- Acceleration and deceleration time
- Jerk or S-curve settings
- Reversal speed
- Approach speed near the end of travel
- Emergency-stop deceleration strategy
- Servo torque limits
An S-curve motion profile changes acceleration more gradually than an abrupt command. This can reduce the peak force transmitted to the nut, shaft-end bearings and coupling, particularly on axes with a large moving mass.
3. Prevent the Carriage from Striking a Rigid Stop
Limit switches and software travel limits should stop the axis before the carriage reaches the mechanical end of travel. A mechanical stop should act as the final safety device rather than the normal stopping method.
A reliable travel-protection arrangement may include:
- Programmed soft limits
- Deceleration switches
- End-of-travel limit switches
- Mechanical stops positioned beyond the normal working stroke
- Industrial shock absorbers where collision energy must be managed
If a shock absorber is used, it should be selected according to the moving mass, impact velocity, impact frequency and available stopping stroke. It should act on the moving structure at an appropriate location-not directly on the ball screw shaft or support bearing.
4. Select the Correct Support-Bearing Arrangement
The support arrangement determines how axial and radial forces are carried. A common fixed–floating arrangement uses a rigid fixed end to locate the screw axially and a floating end to accommodate thermal expansion.
The fixed-end bearing set must match the required axial load, rigidity, speed and accuracy. Simply installing a larger ball screw does not guarantee that the support bearing can withstand the application's peak thrust.
During selection, confirm:
- Maximum axial load in both directions
- Peak load during acceleration and emergency stopping
- Required axial rigidity
- Operating speed
- Bearing arrangement and preload
- Shaft-end dimensions and fits
- Housing and locknut interfaces
DLY provides ball screw support units in fixed- and floating-side combinations such as BK/BF, FK/FF and EK/EF. The appropriate series and size should be selected according to the screw shaft end and machine load requirements.
5. Maintain Correct Alignment
Misalignment does not absorb shock. Instead, it adds radial force and bending to a system already experiencing axial impact.
Check that:
- The fixed- and floating-end supports are correctly positioned
- The screw axis follows the guideway travel
- The ball nut housing is not forced sideways or vertically
- The bearing shoulders and locating surfaces are clean
- The motor shaft and screw shaft are correctly aligned
- The coupling is not being used to conceal a major offset
After tightening the support units and nut housing, move the axis slowly through the complete stroke and check for changes in running resistance.
6. Eliminate Looseness in the Drive System
Loose components can generate a secondary impact whenever the axis changes direction. Inspect the:
- Support-unit mounting bolts
- Bearing locknut
- Ball nut mounting bolts
- Coupling clamping screws
- Motor mounting bolts
- Nut bracket and carriage connection
Fasteners should be tightened to the specified torque and secured using the method required by the design. Excessive tightening is not a substitute for correct fits and locating surfaces.
7. Protect Vertical Axes from Uncontrolled Descent
A vertical axis requires additional attention because gravity continues to act when power or braking force is lost. If the table or load drops, the ball screw and fixed-end bearing may receive a severe impact at the end of travel.
Depending on the machine design, protection may include a motor brake, counterbalance, suitable control logic or another holding device. The selected method must be capable of controlling the actual moving mass under both normal and fault conditions.
8. Inspect the Axis After a Collision
Do not immediately return an axis to full-speed operation after a collision or emergency stop. Begin with a mechanical inspection and low-speed test.
Check for:
- Abnormal axial movement at the fixed end
- Changes in running torque along the stroke
- New noise or vibration
- Heating at the bearing housing
- Movement of the nut bracket or support units
- Coupling deformation or slipping
- Damage to the screw shaft or machined journals
- Loss of positioning accuracy or repeatability
If noise or resistance appeared immediately after the impact, continued operation may worsen the damage. The support bearing, ball screw assembly and associated mounting interfaces should be inspected before normal production resumes.
Lubrication Helps Reduce Wear, but It Cannot Absorb a Collision
Correct lubrication is necessary for the service life of bearings and ball screw raceways. However, lubricant should not be treated as a shock-protection device. It cannot prevent permanent indentation caused by a peak load that exceeds the component's permissible capacity.
Lubrication, sealing and cleanliness should therefore be maintained as part of normal operation, while shock prevention should be addressed through load calculation, motion control, travel protection and mechanical design.
Conclusion
Protecting ball screw support bearings from shock starts with controlling where the impact force comes from. Appropriate bearing selection is important, but it must be combined with controlled acceleration, reliable travel limits, properly selected shock absorbers, correct alignment and secure mounting.
For a new assembly, provide the ball screw model, shaft diameter, lead, travel, moving mass, maximum speed, acceleration, installation orientation and shaft-end drawing. These details help determine whether the ball screw and its support arrangement are suitable for the expected peak loads.
