Beginners Guide

Design Considerations for Idlers

The procedure for selecting idlers for a conveyor for any given application involves the evaluation of a number of factors pertinent to the installation.

The designer must bear in mind that there is a variety of idlers to choose from and that the application may require the use of 'special' idlers.

The following fundamental criteria must be considered by the engineer for each conveyors' idler selection.

a) Design

The design of an idler is undertaken by specialised vendors. Consideration must be given to the following standards and design criteria :-

• Specification / standard
  In South Africa, idlers conform to SABS 1313 which is a dimensional standard allowing interchangeability between various vendors components.
  

• Bearings
  All rollers must be fitted with precision ball or spherical roller bearings. Shafts and housings should be machined to standard I.S.O. limits. One idler bearing should be positively located to the shaft and the other free to float.
  

• Shafts
  Idler shafts must be stationary.
  The shafts should be machined such that the vital dimensions and clearances at the seals are maintained.
  

• Greasing
  Bearing and seal assemblies must be factory-greased and must contain not less than 1 cubic inch of grease for bearing lubrication, i.e. sufficient grease to operate continually for a minimum of six years without attention. "Sealed off" re-greasing facilities can be provided however this should be under advisement from the vendor.
  

• Seals
  Seals should be multi-labyrinth "non-rubbing" type and should prevent the ingress of dirt, air and moisture. The outer seal should be corrosion resistant.
  

• Rolling Resistance
  The total resistance of idler rollers against rotation (measured at the periphery of the idler shell) must not exceed an average of 0.132 kg for medium duty idlers up to 153mm diameter and 0.254 kg for heavy duty idlers up to 178 mm in diameter.
  

• Weather Seal
  Fire and oil resistant weather seals are essential to protect the seal assembly from rain and hosing down operations.
  

• Roller Shells 
  Idler shells should be manufactured from precise tubing selected to within stringent tolerances. Housings and shells should be welded to prevent loosening or misalignment under impact conditions. Rollers should have a corner radius of not less than 3mm over the full 90 degrees of the corners, for protection of the belt.
  

• Balance
  The balance of rollers should be within 7.5 gram/mm on 153 mm and 178 mm diameter rolls and 5.5 grams/mm on 100 mm and 125 mm diameter idlers.
  

• Peripheral Run-out
  Peripheral run-out is not to exceed 0.6mm total indicator (TIR) reading on troughed idlers, with proportionately equivalent lower values for return and other idlers.
  

• Weights
  The weight of the rotating parts which are to be accelerated upon startup of a conveyor is not to exceed the following :-

For light duty 4":	0 (12 to 36" belts)	0.6 Ibs per inch belt width per trough set.
For medium duty 4.5":	0 (18 to 48" belts)	0.7 Ibs per inch belt width per trough set.
For medium duty 6":	0 (18 to 48" belts)	0.8 Ibs per inch belt width per trough set.
For heavy duty 5":	0 (18 to 60" belts)	0.9 Ibs per inch belt width per trough set.
For heavy duty 6":	0 (?? to 60" belts)	1.2 lbs per Inch belt width per trough set.
For heavy duty 7":	0 (48" to 120" belts)	1.4 lbs per inch belt width per trough set.

• Testing
  All rollers should be factory test-run at no less than a simulated conveyor belt speed of 7.62 m/sec to confirm the required TIR, rolling resistance, etc.
  

• Bases
  Idler bases must be of steel construction, designed and machined to accommodate drop-in roller shafts in rattle-free slots. Bases should be free of pockets where material can accumulate and should shed spillage.

IT IS ALSO DESIRABLE THAT IDLER BASES SHOULD BE SHOT-BLASTED PRIOR TO SURFACE TREATMENT, TO ENSURE LONG LIFE AGAINST CORROSION.

• Roller Surface Treatment
  Rollers must be cleaned before being painted with a standard low gloss finish on the ends. Special paint treatment is available to suit requirements. The periphery of steel rollers is to be coated with a corrosion inhibitor. 

b) Installation & Alignment 

Generally the structure carrying the idler must be leveled across the stringers and true along its length. Obviously changes in elevation are certain to occur however these changes must be gradual and smooth.

As the idlers are used extensively to train the belt, it is essential that they are initially installed square to the belt line and horizontal across the stringer. Typically a true structure would require very little belt training thus improving idler life by eliminating the scuffing caused by using the idler to track the belt.

c) Maintenance 

As with pulleys, a well installed idler set will run with little or no trouble for many years.

Idlers fitted with grease nipples should be greased regularly according to a greasing programme developed on site. This greasing programme should be developed in conjunction with the supplier and may differ along the conveyor length. Loading point idlers may require more frequent greasing than return idlers for example.

Damaged idlers should be replaced.

Seized idlers should be removed immediately as they can wear and cut the belt.