For optimum performance and correct running, all chain systems should be provided with means to compensate for elongation due to wear. As a chain conveyor or elevator is a positive form of drive, no pre-tensioning of the chain is necessary. The only adjustment required is the take-up of the clearances between the pins and bushes in each link; this should be done before the conveyor is run.
The amount of adjustment should allow for the joining up of the chain and elongation due to wear. Wherever possible, the adjusting wheel or track should be set at a convenient position following the drive point. This ensures that the effort required to adjust the chain is minimal. Take-up positions should, whenever possible, be introduced at positions where the conveyor makes a 180º bend. At these positions, the chain take-up will be equal to twice the adjustment. If a 90º position is unavoidable, then track movement, particularly on overhead conveyors, will arise. Where multiple drives are used, it is preferable to provide an adjustment for each drive point.
After the chain has been installed and any additional fixtures have been put in place, the tension on the chain should be adjustment, prior to starting the machine. This adjustment should remove any slack from the chain, but should not go as far as pre-tensioning the chain. One of the simplest methods of adjustment is a screw type take up unit, as shown in figure 2. However, care must be taken on these units, as accidently pre-tensioning the chain can reduce the chains life.
As a general guide to adjusting chain;
a) Check that the tail shaft is inline i.e. adjust the bearing slides if necessary.
b) Adjust the take up screws until the chain does not hang from the underside of the sprocket, as shown in figure 3.
c) Lock the adjustment screws into position.
d) Carefully run the unit for a minimum of one complete cycle, allowing the chain and any attachments to settle, before re-checking the chain adjustment.
e) Repeat steps b) to d) if necessary.
Where an automatic take up unit is installed i.e. hydraulic, spring, pneumatic etc. care should be taken to prevent unnecessary pre-tensioning of the chain (see Calculating Tension Loads)
After the chain has been correctly adjusted and lubricated (if applicable), the unit should be allowed to run for 8-24 hours, without load, to ensure that all components can bed in. After this period, the take up should be re-checked and adjusted if necessary. The amount of adjustment varies according to the length and pitch of the chain, which can be estimated as per the left hand table.
When a chain has been adjusted to this extent, it will be due for replacement. Where this calculated figure exceeds twice the pitch of the chain, then a minimum adjustment of plus 1.5 pitches, minus 0.5 pitches on the nominal centre distance should be provided. This amount of adjustment will allow the removal of two pitches of chain as wear occurs; with the minus adjustment providing sufficient slack for the initial connecting up of the chain.
On dredging feed elevators, where the boot wheel is the adjustable member, provision should he made not only for wheel adjustment but also for the lining of the boot so that the buckets remain at a constant distance from the lining.
Figure – Screw type take up unit
Figure – Chain prior (dotted line) and post adjustment
Figure – Adjustment Calculation