Load Chain

 

 

Fine Tolerance – Short Link Chain

·       BS EN 818 – 7, covering: short link chain for lifting purposes — Safety — Part 7: Fine tolerance hoist chain

·       Grades: (Types T, DAT and DT), 4mm to 22mm diameter

·       BS EN 818-7 is applicable to electrically welded round steel short link hoist chains conforming to EN 818-1

·       ISO 3077 - Short-link chain for lifting purposes, - Grade T, (types T, DAT and DT), fine – tolerance hoist chain

Chain Finishes

Fine tolerance chain can be recognised primarily by the Grade mark.

·       Type DAT and type DT hoist chains possess a surface hardness greater than core hardness and are used for power driven chain hoists to offer greater resistance to wear

·       Type DT hoist chain differs from DAT hoist chain in having higher surface hardness and/or greater case depth to optimise wear resistance

Fine Tolerance – Short Link Chain

·       The chain is made to precise dimensions in order that it engages freely, and without jamming into pocketed load wheels

·       The application of a tensile force at the final stages of manufacture (pulled to precise pitch) has the effect of work hardening the chain

·       Fine tolerance chain is less ductile than chain used for general sling manufacture (medium tolerance)

·       Fine Tolerance chain has better wear characteristics

Load chains must:

·       Be strong

·       Be reasonably resistant to corrosion

·       Have good resistance to wear

These properties are partially achieved by material selection.

Further improved by case hardening and/or plated with corrosion resistant finishes at the time of manufacture.

Fine Tolerance Chain

Guidance on the applications for which the different types of Grade T hoist chain shall be used is as follows:

·       Type T manually operated hoists, or power operated hoists with slow speeds, where the working environment does not involve abrasive conditions

·       Type DAT power driven hoists where chain speeds are high in combination with high working capacity and where wear resistance is required to give longer chain life

·       Type DT power driven hoists used in abrasive conditions

Note: Case hardened chains are not suitable in portable manually operated hoists.

WLL Comparison of Chain Grades

Fine Tolerance Chain

Dimensional incompatibility between the hoist chain and mating parts of the hoist (chain wheel, chain guide and loading device) may lead to premature failure of the chain.

BS EN 818-7 contains dimensional requirements for correct assembly and fit.

Key Points

·       Correct alignment with the load wheel

ü Use of suitable chain guide

ü Hoist not to be used if direct entry to load wheel is prevented, or:

ü  Where the chain is twisted

·       Chain must be under tension to engage the load wheel correctly

ü  A few links of load chain will be sufficient

·       Chain should not be corroded or covered in dirt or debris

·       Chain stripper fitted to ensure disengagement of slack load chain from the load wheel

·       Adequate and appropriate lubrication

ü Lubricants should be able to withstand high bearing pressures

ü Colloidal graphite used in adverse working conditions such as foundries, or:

ü Where lubricant may contaminate (e.g., food or pharmaceutical industries)

ü Must be acid free

·       Chain collector box or bag usually fitted

ü Adequate size and alignment

ü Provide a means of drainage

ü Slack chain should never pile too high in the bag/box – this will remove the tension from the slack end of the load chain

ü Could result in a twisted link entering the load wheel and cause disastrous consequences

·       Load chain must always hang in a straight line

ü No twists

ü Load hooks fitted with a swivel to prevent live side of chain from twisting

·       Load chains are never to be back hooked or choked

ü Separate sling attachments

Frequent Inspection

In addition to the requirements for statutory periodic examinations, hoist manufacturers will issue instructions for user inspections. The type and frequency of inspections depends basically on the working conditions of the hoist. General classifications are given for these inspections as ‘frequent' and ‘periodic'.

Frequent inspections are visual inspections carried out by the operator or other designated person, with or without a record being made, to determine if damage or deterioration has occurred in service.

The following inspection intervals are recommended when carrying out frequent inspections and are in addition to a daily pre-use check which should be made by the operator:

·       Light service - every month

·       Moderate service - every 2 weeks

·       Heavy service - every week

·       Very heavy service - every day

Periodic Inspection

Periodic inspections are more thorough inspections by appointed persons making records of the external conditions to provide a basis for a continuing evaluation. The following periodic inspection intervals are recommended:

·       Light service - yearly

·       Moderate service - six monthly

·       Heavy service - quarterly

·       Very heavy service - every six weeks

If at any of these user inspections external conditions indicate it necessary, the machine should be referred to a Competent Person for thorough examination.

Thorough Examination

Thorough examinations are made by a Competent Person, i.e. the tester and examiner. They are usually associated with statutory requirements calling for records to be made and certificates or reports issued which permit the hoist to enter or remain in service. This is far more thorough than the user inspection and will usually include disassembly of parts to permit detailed examination. Each of the regulations lays down a maximum time period between such examinations.

When carrying out thorough examinations the chain should be examined throughout its length to detect any evidence of wear, distortion or external damage. The block should then be operated under ‘no load' and ‘load' conditions in both directions to check for the smooth functioning of the chain and wheels. If the chain jumps, binds or is noisy after cleaning and lubrication then a more detailed examination must take place.

Competent Person – Thorough Examination

·       Preparation for examination

ü Chains should be cleaned (no strong alkalis or acids – hydrogen embrittlement)

·       Visually examine chain throughout entire length, link by link

·       Operate hoist under no-load and loaded conditions

ü Check for directional smoothness

ü Look for chain jumping in the pocket wheel

ü Listen for binding and noisy operation

ü If minor faults are not corrected by cleaning and lubrication, a detailed examination is necessary

·       Link by link examination in adequate light. Check for NCCG (nicks, cuts, cracks or gouges), wear, elongation and other damage including build-up of debris

ü Wear and elongation to be measured in accordance with ‘original equipment manufacturer’ (OEM) instructions

Stretch and Elongation

A common misunderstanding is that stretch in a load chain is the same as elongation. This is incorrect. Stretch in a load chain is not permitted as this is actually the chain having exceeded its elastic limit and now will have permanent set (it is now in the plastic deformation stage) and this should be withdrawn from service. Elongation is wear that has occurred due to articulation of the interlinking chain links at the intrados of the connection point.

·       Manufacturers may have different instructions for measurement and the acceptance/rejection criteria may vary

·       Where there are no OEM instructions:

ü 2% increase in length (due to wear) over a minimum length of 5 links should be used as a rejection value

Note: This is less than 5% which is used for chain slings

·       This is because the load chain in a machine has to mate with the load wheel and therefore elongation would cause a poor fit and dangerous consequences

Some hoist manufacturers issue a gauge for checking of elongation.

The example below is taken from a manufacturers maintenance manual and is typical for most brands of hoists:

 

Chain Elongation due to Wear

Rejection Criteria

Load chains should be rejected if any of the following conditions are observed:

·       Cracks

·       Nicks or gouges

·       Visible distortion

·       Severe corrosion

·       Deposits which cannot be removed

·       Increase in length which exceeds the OEM recommendations or 2% over 5 links

Wear

In the case of wear, rapid wear can lead to sudden failure of the chain. This is shown as a rough appearance of the mating surfaces. Such chains should be replaced even if they are within the OEM wear limits.

Chain Replacement

Calibrated chain for powered lifting hoists varies in dimensions, particularly pitch, for different manufacturers. For this reason it is important that only chain specified by the hoist manufacturer should be used for replacement. Each manufacturer provides details on the best methods of chain replacement for their particular units and it is recommended that these be closely followed.

When replacing worn chain with new chain it is advisable to also replace the pocketed wheel(s) as the pockets will have worn with the chain.

Manufacturer’s Criteria

The certificate of test and examination shall give at least the following information:

·       The name and address of the manufacturer or his authorised representative, including date of issue of the certificate and authentication

·       Number and parts of BS EN 818

·       Quantity and description of the chain of which the test sample is representative

·       Identification of the chain of which the test sample is representative

·       Nominal size of chain in millimetres

·       Manufacturing proof force in kilonewtons

·       Breaking force, in kilonewtons (confirmation of whether this was met or exceeded)

·       Total ultimate elongation at fracture, as a percentage (i.e. confirmation that the specified minimum total ultimate elongation has been met or exceeded).