Idlers Basics

Idlers became a critical component of conveyors as soon as belting was used as a means to support and transport the load.

The fact that idlers come into direct contact with a conveyors' belting extremely frequently, combined with the fact that belting is usually the highest cost item on a conveyor, means that idlers should cause the least possible wear or damage to the belting as possible.

The design of idlers and any improvements made to these designs since their original inception, has therefore usually been driven by the need to reduce the stresses induced into the belting while at the same time, forming and supporting the belt in such a way that the maximum possible load can be transported with the most cost-effective belt width and thickness.

As a result of the above, designers today have a range of idler types, widths, roll diameters and configurations to choose from when deciding on the best idlers for a particular conveyor.

The purpose of this Handbook is to introduce the learner to some of the fundamental considerations which prevail in all troughed belt conveyor applications including an explanation of the terminology of idlers, issues which determine the selection of a particular set of idlers and the industry standards applicable to troughed belt conveyor idlers.

An endless conveyor belt in a conveyor structure is pulled from the tail pulley where material is loaded onto the conveyor, to the head pulley or drive pulley where the material is discharged.

In between the tail and head pulleys, whether the distance is a number of kilometres or merely a few metres, the carrying and return strand belting is supported on idler sets.

- Carry idler sets
- These idler sets support the carrying-side (top) conveyor belt onto which the material is loaded and transported.

- Return idler sets
- These idler sets support the return-side (bottom) conveyor belt which returns to the tail pulley after having discharged product over the head pulley.

Carrying or troughing idler sets usually comprise between two and five individual idler rolls mounted into a common base, which is attached to the conveyor structure.

Each idler roll in a 'set' comprises its own set of bearings, seals, shaft and outer shells as can be seen in the following section.

The design and manufacture of idler roll assemblies, although not a complicated science is nonetheless a specialised field. Many entrepreneurs have entered this field assuming it to be 'basic' and have failed.

The internal components of idler rolls are essentially the same although variations occur from one manufacturer to another. In addition, the quality of idlers also varies depending on the quality of the individual components and of the manufacturing process.

A typical idler can be seen below together with an indication of the individual components :-

The outer shells (3) of idler rolls are generally manufactured from 89, 102, 108, 114, 127, 138, 152 and 178 mm tubing.

The outer shells are generally closed off with an end cap of varying design.(1).

The end caps are normally designed to house the anti-friction bearings (2) and bearing dust seals (5).

The idler shaft has special machined ends to allow its positive location within the base frame idler support brackets.

An exploded view showing the components and assembly of a Melco type idler can be seen by clicking on this thumbnail.

1	Plastic Grease Plug--Coloured for grease type and year of manufacture.
2	Weather Seal - Non-rubbing, water and fire resistant.
3 & 5	Outer Seal Components - Multi-labyrinth non-rubbing close rubbing (Press fit seals)
4 & 6	Inner Seal Components.
7 & 11	Circlip - Locates bearing on shaft (one end only), spring steel hardened and tempered.
8	Bearing - Deep groove single row precision ball or spherical roller, single row C3 & C4 clearance.
9 & 10	Rear Seal Assembly--Combination pressed metal/plastic, non-rubbing seal.
12	Identification Slot.
13	Shaft Ground shaft accurately located circlip grooves.
14	Shell and end, deep drawn steel pressing and concentric bearing housing bored with .001 inch tolerance.
15	Corner Radius - Generous radius on corner of roller, essential to protect belt.
16	Rigid Shell- Small runout.
17	Breather hole.

In addition to the idler roll itself, the idler base and brackets. These bases and brackets locate the individual idler rolls relative to each other and provide the necessary 'trough' angle.

Examples of standard idler support frames and brackets are provided, courtesy of Melco, which can be viewed by clicking on the applicable thumbnails :-

The idler brackets and bases incorporate slotted holes for which coincide with the ____ on the ends of the roll's shafts. This facility enables worn or damaged rolls to be replaced relatively simply.

Whenever the details of idler rolls, bases and brackets are discussed by designers, it is important that the terminology used is clearly understood.

- Standard idler rolls
_________	=	Roll diameter	PIC
_________	=	Roll face length
_________	=	Shaft diameter
_________	=	Shaft length
_________	=	Guage length
_________	=	Slot width ??????
_________	=	_________

- Standard troughed idler bases
_________	=	Pitch of securing bolts	PIC
_________	=	Width ________
_________	=	Height ________
_________	=	_________

- Standard flat return idler brackets
_________	=	Drop height	PIC
_________	=	_________
_________	=	_________
_________	=	_________

- Standard troughed idler bases
_________	=	Drop height	PIC
_________	=	_________
_________	=	_________
_________	=	_________

In addition to the above, idlers are divided into different 'series' according to their shaft diameters. Series 25, 30, 60 etc. idlers thus refers to idlers which have shaft diameters of 25 mm, 30 mm and so on.

It has been stated above that there are basically two types of idlers, carrying and return idlers.
Within these categories however, there are a number of different designs of idler sets which have developed as a result of particular applications, the need to minimise the stress imposed onto the belt and the environment in which the conveyor operates.

For the learner, these different types of idlers are described below, together with a functional explanation for each.

Troughing idlers are found on the carrying-side, along the length of the conveyor. On any particular conveyor these idlers are identical, as are the bases.

The troughing idlers comprise a centre idler roll of a defined width, and 'wing' idlers on either side of the centre roll.

Unlike the centre, horizontal roll, the wing idlers are cranked up to a predetermined angle known as the troughing angle.

This troughing angle ensures that the carrying belt maintains the same cross-sectional area throughout the carrying strand, so that the load-bearing capacity of a particular conveyor belt is the same along the conveyors' full length. In so doing, material loaded to the maximum capacity at the loading point will not fall off of the belt en-route.

The 5-roll idler configuration offers a more uniform cross-section which results in a greater nett carrying capacity for the same belt width and belt speed.

The garland idler set (see thumbnail right) comprises either 3-roll or 5-roll idler sets joined together at the ends of their respective shafts by special lugs, to form a continuous 'chain' of idlers.

On either end of this 'chain', the shafts are connected to an attachment bracket which enables the idler set to be attached to the conveyor stringer structure.

Garland idlers are used predominantly on yard conveyors, shiftable conveyors, and dump conveyors, etc. where the alignment of the conveyor structure may not be completely correct due to the mobility of the conveyor, or the ground conditions.

The garland idler design is more forgiving than the fixed-base troughing idler, due to its chain-like design which allows the belt centerline and idler centers to move relative to the supporting frame.

Transition idlers are found at either end of the conveyor, adjacent to the head and tail pulleys.

These idler sets comprise standard idler rolls however the bases into which these rolls are fitted, have a smaller troughing angle to that of the rest of the troughing idlers on the conveyor.

The reason for this is that the conveyor belt is flat as it passes over the pulleys, under high tension. As the belt changes its' form to a full trough of say 35 degrees (i.e. from tail pulley to full trough angle), the belt must be supported through this transition zone. If the belt was fed directly into a full trough from the tail pulley, the belt edges would be over-stressed and damage would result.

The number of transition idlers depends on the trough angle of the conveyor. In the case of a 45 degree trough angle, 2 or 3 transition idler sets would be used at either end of the conveyor. These idler sets would have incrementally greater trough angles of say 15, 20 and 35 degrees through the transition zone leading up to 45 degrees.

As with troughing idlers, transition idlers are available in fixed bases or the garland configuration.

Wherever material is loaded onto a conveyor belt, impact idlers are installed beneath the troughed belt over the full loading length.

Impact idlers sets are spaced at close intervals of typically 350 mm to 450 mm in order to provide a comprehensive support base for the belt.

Impact idlers comprise a small diameter shell, typically 89 mm diameter, fitted with concentric rubberized rings at close intervals. These rubber rings provide a cushioning effect in the event of lumps of material falling onto the belt where an idler is located beneath the point of impact.

This cushioning ability of impact idler sets allows the energy associated with the impact to be absorbed more efficiently and with much less detrimental effect to the belt.

Impact idler rolls are either mounted into rigid, fixed base frames or can be manufactured in garland-type sets, depending on the application.

The mass of the return belt is the only load that return idlers are required to support. As such, return idler sets are spaced at two to three times the pitch of their equivalent carrying-side idler sets. Return idler sets also usually only have one or two rolls per idler set, for similar reasons.

There are of course exceptions to the above, one being where conveyors carry material along the top and bottom strands as would be the case on two-way conveyors. In this case, troughing idlers would be located along the bottom strand as well as the top strand.

Flat return idlers comprise 1 or 2 rolls, the 1-roll design being more prevalent. In the case of a single flat return idler, the idler can be mounted onto two brackets which are secured to either side of the conveyor support structure as seen adjacent.

Where flat return idler sets comprise two flat rolls, a support frame is required which supports the idler pair and attaches the idlers to the conveyor frame. This 2-roll design is not a common design unless the return idlers are formed into a trough in which case the return idler sets are referred to a 'V' return idlers.

The trough angle of the 'V' return idler is usually 10 degrees and this feature has been developed to assist with belt training along the return strand.

in addition to the fixed base frames for return idlers, garland-type return idlers are also available which, like their equivalent troughing idlers, are more tolerant of conveyor structure misalignment.

In spite of the correct alignment of a conveyor structure and belt splice, it is a common phenomenon for a belt to misalign at times, mainly as a result of incorrect material loading (off centre) or incases where a conveyor is mounted onto a mobile stacker / reclaimer where the fall across the boom causes the belt to move to one side.

In cases where the belt misaligns, severe damage can be done to the belt should the belt rub against a fixed structure, the head chute or conveyor stringers.

A solution to belt misalignment can be to install self-aligning idlers sets or training idlers which are able to detect misalignment and automatically re-align the belt.

Training idlers work on the principle that to align a belt the idler set must be turned slightly off of the centerline of the conveyor, so as to cause the belt to track back to the correct alignment.

There are a number of different types of training idler designs available in the market which can be used on the carrying and return strands of the conveyor.

Examples of training idlers are shown adjacent and can be viewed by clicking on the different thumbnails.
The idler rolls used in training idler frames are usually standard rolls used elsewhere on the conveyor.

The majority of idlers comprise steel shells which are fabricated from standard carbon steel tube sections.

A range of materials are however available for shells and depending on the application. Usually users decide on the preferred shell material based on the materials handled on the conveyor and / or the environment.

Examples of materials of manufacture of idler shells other than steel and rubber discs (include PVC, Stainless Steel, ...etc.