Installing Ball Screw Support Bearings...and the Biggest Mistake People Make
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What is a Linear Axis?
Linear axes in CNC and machine tools - consisting of linear guides and screw drives - are complex, highly engineered, and balanced system that make our lives easier when they are maintained properly, and turn into nightmares when they are serviced incorrectly. The symptoms are nearly endless:
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Rough travel over stroke
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Binding and overload on the motor
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Positional error
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Rough operation with charter marked parts
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Machine crashes
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Bearing failures
If you’re experiencing these issues, it’s possible that you, your CNC service tech, or your maintenance team is making a mistake with just one component: the ball screw support bearing. Ball screw support bearings look like any standard ball bearing off the shelf, but they have one not so commonly known feature: operation under preload.
What is preload?
Preload, in mechanical terms, is a compressive or tensile force applied to a system or component at installation and before forces of operation. Preload serves a variety of purposes including changing resonance, increasing strength, and mitigating buckling.
How is preload used in a machine tool or CNC linear axis?
1) Preload is usually used in these types of tools to increase rigidity and decrease deflection. Main spindles use clearance-free super precision bearings that are preloaded with offset spacers or grinding on the bearing. 2) Linear profile rail guidance systems use preload between the guideway (or rail) and the carriage, usually by sorting and oversizing rolling elements - balls or rollers - to reduce deflection. 3) Pretensioned screw drives - ball screws or roller screws - stretch the screw to reduce deflection that would cause error between input and position.
And finally: 4) ball screws supports use offset grinding of bearing rings to set preload in a bearing. This “hard stop” prevents over-torquing and damaging the bearing. Ball screw supports look like standard ball bearings, but their precision - specifically relating to axial runout and preload - are closer to super precision spindle bearings.
So if you find the torque values in the catalog and a torque wrench in the shop, you should be good to go? Not quite. Installation issues and errors arise from shops or techs that are insufficiently trained.
Instructions for Setting Torque on Ball Screw Support Lock Nut:
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Identifying the proper tightening torque M_A and available locknuts
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Using the correct precision locknut and wrench/socket
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Don’t set the assembly by feel
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Negate the effects of settling by going to 2 times (200%) final torque before setting.
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Tighten the locknut set screws to 0.5 x M_m and then full M_m torque.
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Identifying the proper tightening torque M_A and available locknuts - use the internet or a manufacturer’s catalog to identify the bearing you are mounting. Sites like Schaeffler’s Medias online catalog provide all the relevant features of the bearing, the GD&T tolerancing of the adjacent construction (shaft and housing), as well as everything you need to mount the bearing. For example, the INA ZKLF40100-2RS-XL screw support bearing Medias page shows drawings of the internal and external construction, provides the correct fixing screws for the flange, and also specifies the use of a ZM40 or AM40 locknut. When using either locknut, the final torque spec M_A to set the the assembly at is 55 Nm (40.57 ft-lbs).
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Using the correct precision locknut and wrench/socket - precision locknuts that utilize axial or radial clamping for positioning and setting torque are specifically designed for high precision application like this. ZM series locknuts with radial clamping use radial compression on the thread via locking pegs. For higher axial load application, ZMA series locknuts use a heavier pegs and a have larger OD for increase strength. AM series locknuts with axial clamping provide axial compression of threads to increase the axial force they can handle and feature higher breakaway torque thanks to increased surface area contact. Low precision locknuts or cheap imitations can great reduce strength, rigidity, and breakaway torque making the assembly less precise and more prone to failure. A jam nut (two single nuts back to back) should not be used because it’s nearly impossible to set final torque - too high or too low of torque settling can cause damage to the bearing assembly that leads to rapid deterioration and ultimately failure and machine downtime. Nuts and threads with holes for cotter pins are even worse, but have been designed in my engineers who didn’t read the instructions or this blog post.
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Don’t set the assembly by feel - the precision of the bearing and associated locknut allows a very precision torque input to the bearing and resulting clamping force. Installers familiar with installing tapered bearings in wheel hub assemblies usually set internal clearance in the assembly by “feel” - taking the nut to snug and backing it off a quarter turn. This is right for an old truck, but incorrect for a machining center. Ball screw support bearings of all designs - two single row bearings like BSB/7602/7603/TAC, double row combinations like ZKLN and ZKLF, matched pair combinations 2AP, and ZARF and ZARN screw supports - have a specified precision locknut with a specified tightening torque.
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Negate the effects of settling by going to 2 times final torque before setting final torque - now finally, the last piece of the puzzle hidden deep in the installation manual, is the proper process for setting torque in screw support bearings. Instructions recommend lightly oiling the thread before screwing the locknut firming against the inner ring of the ball screw support bearings to aid with applying torque. Now, with either the screw fixed against rotation or the locknut fixed, tighten the 2 times (2x) the tightening torque M_A before loosening again. Going back to the INA ZKLF40100-2RS-XL in Step, we should take the ZM40 or AM40 locknut to 200% or 2 x M_A = 2 x 55 Nm = 110 Nm and then loosen the locknut back to our starting point. We can then go to 100% M_A or 1 x M_A = 1 x 55 Nm. If we’re using the ZM precision locknut with locking pegs that radially contact the thread, again the catalog specifies a tightening torque for the locking pegs, at 5 Nm for ZM40. For this torque, tighten one of the locking pegs to 0.5 x M_m, switch to the opposite locking peg and repeat at 0.5 x M_m (50%), then take first peg to 1 x M_m (100%) and repeat for the second.
That’s the secret...
The published, but well hidden, secret of torquing precision locknuts on ball screw supports. This applies to INA ZKLF, ZKLN, ZKLR, DKLFA, and ZKLFA bearing assemblies as well as single row FAG BSB/7602/7603 screw support bearings. We referenced the INA ZKLF bearings and their manufacturer Schaeffler’s resources, but the basic principles are suitable for bearings from most precision bearing manufacturers - provide your bearings numbers and we can build a step by step procedure just for you. NSK TAC and Nachi’s TAB ball screw supports are single row axial angular contact bearings like the FAG BSB/7602/7603 series and their assemblies use the same basic instructions.
Resources
Have you been following these steps all along? Does this address some of the issues you’ve seen in your equipment? Drop us a line in the comments below or reach out to use on Twitter or Instagram.