This guidance provides information regarding the repair of fork arms of fork-lift trucks (FLT), and alerts all to certain bad practices which may cause FLT arms to fail. BS ISO Standard recommends that repairs are carried out by the manufacturer or someone of equal competence.
The current relevant BS ISO Standards for fork arms for fork-lift trucks are as follows:
BS 5639-1: 1978, ISO 2331: 1974 Fork arms for fork-lift trucks Vocabulary for hook-on type fork arms.
BS ISO 2328: 2007 Fork-lift Trucks. Hook-on type fork arms and fork arm carriages. Mounting dimensions.
BS ISO 2330: 2002 Fork-lift trucks. Fork arms. Technical characteristics and testing.
BS ISO 5057:1993 Industrial trucks. Inspection and repair of fork arms in service on fork-lift trucks.
Part 5: 1978 Guide for inspection and repair of fork arms in service.
GN 62 – BITA Guidance – Maintenance, inspection and repair of fork arms and attachments.
3 BS ISO 2330: 2002 is a performance standard, and although it does not specify materials, method of manufacture or heat treatment etc. it does however, specify that fork arms should be capable of withstanding 3 times their specified capacity without permanent deformation.
Manufacturers therefore make their own decisions on how to meet this performance, and select materials, manufacturing methods etc, accordingly. The steels from which fork arms are made will thus cover a wide range of carbon and alloy steels. The method of manufacture will also vary in that the top hook may be forged integral with the shank, or may be welded on subsequently. Bottom hooks are usually welded on.
BS ISO 5057: 1993 gives recommendations on the inspection of fork arms, and deals with surface cracks, difference in height of fork tips, positioning lock, wear in fork arm blade and shank and fork arm mountings. It also states that only the manufacturer or an expert of equal competence shall decide if a fork arm may be repaired for return to service. A common reason for the rejection of fork arms is that wear of the heel has exceeded the 10% of the original thickness permitted by BS ISO 5057: 1993. This wear is usually caused by the forks rubbing along the ground possibly as a result of failure to adjust the truck load chain(s) to provide the necessary clearance.
The reason for the BS ISO 5750 recommending that repairs are only carried out by the fork arm manufacturer or an expert of equal competence may not be clearly understood by the truck owner. If welding is to be carried out, for example, to replace a top hook, the repairer should be aware of the steel specification from which the original components were made. The correct material for the replacement part, the correct welding consumable and the correct welding method can then be selected. The welding method will include weld preparation, pre-heating if necessary, stress-relieving if necessary and re-heat treatment to the manufacturer’s specification. Use of “mild steel” materials and ordinary jobbing welding methods are likely to result in an unsatisfactory and unsafe repair.
It should be noted that the BS ISO 5057 considers that surface cracks and wear are not suitable for repair by welding. Reputable repairers do not recommend welding at the heels of forks to replace metal removed by wear, as this will only replace the thickness, not the strength, and may do further harm by mis-matching of materials, localised heating, lack of heat treatment etc.
After welding repairs, re-setting etc, the BS ISO 5057 recommends that the fork arms are tested to 2.5 times their capacity.
Enquiries by HSE Mechanical Specialists have shown that many repairers have no understanding of the metallurgical welding and heat treatment aspects, and are applying village blacksmith methods to such repairs. While badly-repaired fork arms may achieve the 2.5 times proof load, the method of repair is likely to cause detrimental long-term effects which may lead to sudden failure of the fork arm in service.
Article source: http://www.hse.gov.uk/workplacetransport/lift-trucks/repairs.htm