Analysis of the Causes of Ball Screw Clearance
Ball screws are precision components widely used in industrial machinery and equipment, playing a vital role in converting rotary motion into linear motion with high accuracy and efficiency. However, a common problem that affects its performance is the clearance or backlash between the ball nut and the screw shaft. This article will delve into the causes of this backlash.
1. No or insufficient preload
Non-preloaded ball screws will slide down when placed vertically with the nut rotating due to its own weight; and there will be considerable backlash, so they can only be used in machines with low operating resistance, with special consideration given to the requirement for positioning accuracy. Different applications determine the correct amount of preload and adjust the preload before shipment; therefore, when you order ball screws, please describe the operation of the equipment.
2. Too much torsional displacement
1) Improper heat treatment, too thin hardened layer, uneven hardness distribution, or too soft material
The standard hardness of steel balls, nuts, and screws are HRC 62~66, HRC 58~62, and HRC 56~62 respectively.
2) Improper design
The slender ratio is too large, the smaller the length and diameter ratio of the screw the higher the rigidity, the limit of the slender ratio must be below 60 if the length and diameter ratio is too large the screwball will produce the sagging of the self-weight.
3. Improper bearing selection
Usually, industrial ball screws must be used with angled bearings, especially those designed with high-pressure angles; when the precision ball screws are subjected to axial loads, general deep groove ball bearings produce a certain amount of axial backlash, so deep groove ball bearings are not suitable for this purpose.
4. Improper bearing mounting
1) If the bearing is mounted on the ball screw assemblies and they do not fit perfectly that the backlash will come for axial loads, which may be caused by too long or too short shoulder of the screw.
2) Poor perpendicularity can lead to misalignment between the bearing seat surface and the axis of the locking nut's V-groove, or poor parallelism between the locking nut’s surface in two corresponding directions, resulting in skew. Therefore, to ensure perpendicularity, the locking nut’s V-groove at the shoulder and the bearing seat surface should be machined simultaneously, with grinding processes being the preferred method.
3) Two locking nuts with spring washers are used to secure the bearing to prevent dislodgement during operation.
5. Insufficient rigidity of nut seats / bearing seats
If the nut seat or bearing seat is not sufficiently rigid, it will deflect due to the weight of the element itself or the load of the machine.
6. Improperly assembled nut seats / bearing seats
1) Element dislodged due to vibration or unsecured pin. Replace spring pins with solid pins for positioning purposes.
2) The fixed screws of the ballscrew nut cannot be tightened because the fixed screws are too long or the screw holes of the nut are too shallow.
3) The fixed screws of the nut come loose due to vibration or lack of spring washers.
7. Surface parallelism or flatness of the screw support is out of tolerance
Regardless of whether the surface of the bonding element is ground or scratched, as long as its parallelism or flatness is out of tolerance, the reproduction accuracy of the position during the movement of the bed table will be poor; therefore, in a machine, a thin spacer is usually used between the support base and the body of the machine to achieve the purpose of adjustment.
