Wednesday, November 25, 2009
Principles of Ballscrew Repair
Principles of Ballscrew Repair
What exactly does happen to a ballscrew when it fails and what are the principles and procedures to repair a ballscrew from a company such as Jena Tec?
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 decceleration 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
A - Metal Fatigue & Wear to Bearing Surfaces
B - Surface Brinelling
C - Wear & Damage to Balltrack
D - Loss of Squareness to PCD
E - Ball Wear/Breakage
F - Loss of Pre-Load
G - Shaft Bent
H - Damaged Threads
I - Uneven wear over Balltrack
J - Poor System Concentricity
K - Wiper Seals Worn Leaking Lubricant
L - Excessive Backlash
M - Return Mechanism Worn/Broken
The good news is that 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. Firstly, 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 healthcheck to check the major points contributing to the ballscrew performance. You can see some of the major areas of inspection referred to above.
After extensive cleaning and checking, 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. Based on Jena Tec's expert assessment, the unit is warrantied with recommendations for the customer on the extended life period.
While many claim to offer ballscrew track re-grinding, Jena Tec advise 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.
What happens when the 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's extensive stock of precision ground, rolled and whirled product means that we can offer a matched replacement sometimes within the same working day to provide an OEM quality replacement ballscrew.
This means that Jena Tec has the ability to support not only your repair requirements but also your spare parts backed up by our highly trained linear motion technicians, high stock levels and capable precision machine shop.
Typical makes and types of ballscrew repaired by Jena Tec include;
THK NSK Steinmeyer Mannesmann
Hiwin PMI ABBA Comtop
20th Century TCM Blis Berger Bosch
SKF PSI Saginaw Nook
Rockford Nachi Thompson IBL Star
Shuton Iperanga Korta LSI
Barnes American Ballscrew Warner Electric
For service and support on design, selection, repair and replacement of ballscrews and linear motion from Jena Tec go to www.jena-tec.co.uk or call our UK office on +44 (0)1623 726010