Conveyor Chain Wear
Stephen Maw reports on the use of some very practical solutions to reduce chain wear problems encountered at a parcel handling and sorting facility
At a sorting facility where parcels are transported on trays attached to two conveyors, wear problems were being experienced. Each 350 metres long conveyor consists of a length of chain which has alternate horizontal and vertical axles allowing articulation in both planes. Guide rollers are attached at each end of the horizontal and vertical axles to provide location and the vertical axles have an additional roller through which the conveyor is driven. There are three drive units positioned around each conveyor circuit delivering approximately 3kW per drive.
Excessive wear was experienced after 12 months operation, so a detailed review was undertaken by Independent Consulting Engineers.
Investigations confirmed that the conveyor chains were experiencing considerable wear to both the steel axle and nylon bush. The vertical joints had experienced more wear than the horizontal joints, due to the greater articulation that takes place during the operational cycle. The maximum amount of wear measured at a vertical link was 0.6mm (combined axle and bush wear) with an average value of 0.4mm per link. The horizontal joints had worn less than 0.1mm in all cases.
In the case of the vertical joints, more than 50% of the wear had occurred on the steel axle. When dismantled, the worn chain links were seen to be coated with corroded (oxidised) steel wear particles from the worn axle. These particles had acted as an abrasive causing an acceleration of the wear rate of both the axle and the bush. Based on theoretical wear performance of nylon, calculations showed that the chain would be expected to last for approximately five years. However in this case the chains had worn five to 10 times faster than would have been expected.
Detailed recommendations were made for the material composition and properties for both axle and bush to reduce wear by a significant factor:-
- Axle: 13% chromium steel 420 S 37 (En56C), which has improved corrosion.
- Bush: Use reinforced polyester resin, which has a lower wear rate compared to nylon. Clearance 0.1mm diametral between axle and bush.
- The use of proprietary material was estimated to give an improvement in wear life of between five to 10 times that of nylon; however nylon would give a more economic solution.
The modified chains were then manufactured and put into service approximately four years ago. To identify how the chain has performed since then a recent project was undertaken to inspect the chains and assess performance in terms of wear.
Wear had occurred to the nylon bush, but significant improvement has resulted from the change in materials. In particular the use of the 420 S 37 steel for the axle has resulted in negligible wear of the axle. A maximum of 0.24mm wear was measured on one bush after four years in operation which compared to the worst link of the previous design, represented an improvement of five to 10 times (previous maximum wear was 0.595mm in one to two years).
In summary, the measurements of wear show that the chain inspected has lasted four years and has reached the limit of operation based on the angular movement of the tilt trays. However, the life could be extended by moving the position of the trays by one or two links which would give maybe another two years of operation. Occasional application of a lubricant every six months (consult chain manufacturers for lubricant recommendations) may prolong the life of the chain.
Stephen Maw MA CEng MIMechE
Managing Director – Neale Consulting Engineers Limited, Farnham