Beginners Guide

Backstops

Inclined conveyors require an anti-runback device to prevent reverse movement of the belts. Such a device is referred to as a backstop, or holdback. Though backstops are most likely to be found on inclined conveyors, they are also employed on flat, overland conveyors to avoid the unusually severe shock loading on start-up where the loaded belt sags between idlers. This section will direct its attention to backstops installed on inclined conveyors.

Without a backstop, a reversing conveyor can rapidly accelerate to a runaway condition, which can kill or injure personnel, damage or destroy drive train components, tear or rip expensive belting, or cause considerable other damage. A backstop is essentially a safety device which acts to prevent reversal thereby protecting against any of the above from occurring, as well as the massive clean up of material spillage than can occur.

Backstops can be classified either for low-speed or high-speed use. Consulting engineering firms generally specify the use of low-speed backstops on all inclined conveyors where the motor power exceeds 30kW to 40kW.

Low-speed Backstop Design Types

There are three basic backstop designs that are or have been used to prevent anti-runback throughout the many years of conveying materials;

1. rachet and pawl

2. differential handbrake and

3. the overrunning clutch design

The advantages and disadvantages of these units is best shown in the table below:

	Rachet and Pawl	Differential Handbrake	Over-running
			Roller	Sprag
Subject to wear	YES	YES	NO	YES
Affected by dirt	YES	YES on most	NO	NO
Requires adjustment	NO	YES	NO	NO
Backlash	YES	YES	NO	NO
High Stress Concentration	YES	NO	NO	NO
Price	LOWER	LOWER	HIGHER	HIGHER

Low-speed Backstop Design Types

The over-running clutch type backstop is designed for precision operation, automatically engaging to transmit torque when relative motion is in the driving direction and freewheels when relative rotation is in the opposite direction. This design provides a wider operating speed range than other types of backstops and much greater torque ranges - in excess of 700,000 Newton-metres.

There are two basic types of over-running clutch style backstops; roller on inclined planes and sprag clutches.

Roller-Clutch

The roller on inclined plane design consists of two concentric races, one cylindrical (the outer race) and the other precision machined with a series of inclined planes or wedge-shaped surfaces equally spaced around the circumference (the cam). Precision ground rollers are installed between the inclined planes and cylindrical race and it is the wedging action of the rollers between the two surfaces that transmits the torque. The rollers are separated from both surfaces by an oil film during freewheeling so no wear occurs in this mode of operation. When the clutch slows down as the pulley shaft decelerates, the spring loaded rollers overcome the viscous shear of the oil bringing the rollers up the inclined plane to insure automatic backlash free engagement when the pulley shaft stops and tries to reverse.

Sprag Clutch

The sprag clutch design consists of circular inner and outer races and a complement of non-cylindrical, irregularly shaped wedging elements or sprags. The sprags are installed in the annular space between the two cylindrical races. During freewheel, the sprags must be retained in position to engage, so these elements rub on the races. Since a backstop freewheels most of the time, this constant rubbing of the spring loaded sprags will produce wear both on the races and the sprags. When the sprags rotate to wedge between the races to transmit torque, they always engage on the same contact point of the sprag, unlike a roller which has an infinite number of points of contact.

Location for Backstop Installations

A low-speed backstop generally refers to units that are running at conveyor drive pulley speeds. Most frequently, low-speed backstops are mounted directly on the extended head pulley or drive pulley shaft opposite the drive, as shown below.

 

This provides the most positive means of controlling belt reversal. Further, it also allows necessary service work on the drive components (i.e. reducer, couplings and motor) to be performed with ease as no reverse torque is present.

If space or some other factor is a problem for locating as above, then an alternative location would be to mount the backstop on the double extended low-speed reducer shaft. Mounting the unit in this fashion does subject the backstop to the inherent vibration in the reducer shaft together with higher operating temperatures. Either of these conditions could increase maintenance on the backstop. This does provide a more convenient location for servicing the unit than if it were located between the pulley shaft bearing and the low-speed coupling. With the backstop mounted on the reducer, should a failure occur in the low-speed coupling, the conveyor would run back as the backstop would not then be connected to the pulley shaft.

Backstop Size Selection

The vast majority of recognized engineering firms, both in the US and abroad, use the breakdown or stalled torque of the driving motor(s) to size the backstop. We concur completely with this method of sizing. This ensures that the backstop will not be damaged in the event the belt becomes jammed or stops due to an overloaded condition. Since there is no backlash in the backstop, the torque that it must be capable of withstanding is the equivalent breakdown torque of the driving motor(s) at the head shaft which will be present in the system as stored energy or rubber band effect.

Most manufacturers of low-speed backstops have a prescribed method for size selection based on the torque rating of the backstop in conjunction with a recommended service factor to be used based on the maximum torque characteristic of the driving motor(s). The accompanying table shows typical service factors to be applied when size selecting backstops. Torque rating of the unit would include a motor torque characteristic of 175% as built in; i.e. a service factor of 1.0.

Table

Maximum Breakdown	Service
or Stalled Torque	Factor
% of Normal Motor Rating
______________________________ ______________________________
175%	1.00
200%	1.15
225%	1.30
250%	1.50

Backstop Design

There are several types of backstops available. The principle ones are the friction and interference (cog) types. The cog type backstop does not lend itself to load sharing analysis which is the main item of concern.

An example of the friction type backstop is shown in Fig. 3.

Figure 3

This type of backstop operates by wedging the rollers between the inner cam ramps and the outer race when in the backstop mode. The energising springs are under tension at all times even when the backstop is free wheeling. The spring force keeps the rollers in constant contact with the ramp and outer race surfaces for instantaneous response when free wheeling ceases and backstopping commences. The backstop manufacturer is expected to provide prudent engineering judgement in the selection of materials, specifications for allowable stresses, strains, deflections and manufacturing accuracy. All element functions must interact smoothly whether in the free wheeling or backstop mode. Since most backstop failures occur when overrunning, the fits and clearances must be carefully analysed for all conditions of operation. The interaction of the loaded and deflected backstop components is of primary importance when considering the sharing of load between rollers. The simultaneous engagement of all the rollers in the backstop design shown on Fig. 3 is probably not attainable in practice even with the most stringent requirements for manufacturing accuracy. However, machining of parts to nominal tolerances is acceptable because as the backstop load cycle develops, stressed and deflected parts make elastic accommodation to one another which diminish, stress. Example: The ratio of the first roller load to the last roller load at the start of engagement may be very high, however, with increasing load the difference declines rapidly so that at approximately 20% of rating the difference is negligible. Fig. 4 shows the point at which rate of elastic deflection and the rate of elastic plus manufacturing tolerance deflection (inelastic deflection) are the same on a torsional deflection test of an individual backstop. The importance of this elastic deflection will become apparent later. In any case, it is evident that manufacturing accuracy significantly affects the backstop response.

Figure 4

Important features of any design are those that reduce stress concentration and load distribution of contacting loaded members. In the design shown, Fig.5 , note the outer race flange not only strengthens against a "bell" effect at the outer race ends, but also removes the side plate fastening holes from the stressed outer race mid-section as is done on some designs. Localised stress, 2 to 3 times greater than nominal can be created by the hole stress riser effect.

Figure 5

The materials selected must be able to withstand substantial roller to ramp and roller to outer race contact stress. Nominal contact stresses (Hertz) are in the 450,000 to 550,000 psi range with surface indentation (Brinelling) starting at approximately 650,000 psi up to 750,000 psi. These parts are normally case carburized or flame hardened to a depth which depends on load and dimension of contacting elements. Typically, hardness is 56 to 65 Rc.