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Jena Rotary Technology (Jena-Tec) outlines what happens to a ballscrew when it fails and the principles and procedures involved in repairing ballscrews.

The most frequent cause of failure of a ballscrew is either loss of pre-load or rough running typically characterised by a fault flagged by a CNC control system of a machine tool or dimensional inaccuracy or poor surface finish.

This in turn is caused by gradual degradation of the ballscrew surface through metal to metal contact of the re-circulating balls to the hardended metal surface of the ballscrew shaft.

This repeated contact over an extended period of time leads to a worsening of the ballscrew surface and ultimately erosion and wear.

Lubrication can therefore have an obvious impact on the lifetime of the ballscrew, as well as protection from debris from the machining processes.

Over time, as a machine condition worsens, axis covers and bellows can be damaged and allow machining debris to enter the ballscrew nut even through wiper seals.

Over-lubrication can also be problematic, particularly with grease-lubricated systems where too much lubrication can cause pressure on the ballnut-return system, causing the tube to literally burst with excess grease.

The second most frequent cause of ballscrew failure is catastrophic failure due to an exceptional shock load, which is more commonly recognised as a machine crash.

The abrupt excessive load and rapid deceleration from a machine crash can cause buckling of the ballscrew shaft and damage to the ballnut itself, including splitting some of the re-circulating balls within the ballnut, meaning even if the machine runs afterwards the ballscrew will very quickly fail after a short period of running.

Typical ballscrew failure points are: metal fatigue and wear to bearing surfaces; surface brinelling; wear and damage to balltrack; loss of squareness to PCD; ball wear/breakage; loss of pre-load; bent shaft; damaged threads; uneven wear over balltrack; poor system concentricity; wiper seals worn leaking lubricant; excessive backlash; and return mechanism worn/broken.

For routine wear, a ballscrew life can be extended for a further period of service by a very straightforward procedure of repair of the ballscrew by a company such as Jena Tec.

First, the ballscrew is checked for its initial condition, this includes checking its free running condition and the ‘as received’ pre-load or drag torque.

The ballscrew then undergoes a 32-point health check to inspect the major points contributing to ballscrew performance.

The ballscrew, if repairable, is polished along the length of the ballscrew track and undergoes ultrasonic cleaning of the ballnut coupled with high-pressure washing to remove baked-on grease and grime.

The ballscrew assembly is then re-built with oversized micron-graded precision ball bearings to achieve a satisfactory level of pre-load along the length of the ballscrew according to DIN specifications.

The unit is then put on extended test and Jena-Tec facilities include the ability to document and test the ballscrew running performance on motorised testing rigs.

Where a ballscrew passes, a complete documented test report is provided for the customer detailing the received condition and the pass criteria with drag torque graphs and certificates of conformance.

The unit is warrantied with recommendations for the customer on the extended life period.

Jena-Tec advises that due to a weakening of the induction-hardened surface and also the cost to re-grind, that in most cases it does not make economic sense to re-grind the ballscrew track form.

It is however feasible to re-furbish bearing end journals by hard chrome plating or metal spraying and then re-grinding with a precision universal grinder to exacting bearing fit tolerances.

Fixing thread forms are always checked, cleaned and re-furbished where possible, as well as wiper seals, fixing key ways and a variety of related small details.

If a ballscrew is not in a repairable condition, not all is lost as most ballscrews can be repaired to give a shorter extended life, but in this instance Jena-Tec would not offer a warranty but strongly recommend the customer buys a new replacement unit.

Jena-Tec has a stock of precision ground, rolled and whirled product means that can offer a matched replacement, sometimes within the same working day, to provide an OEM quality replacement ballscrew.

Jena Rotary Technology

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