In today’s longwall mining environment, any downtime with the face conveyor sprockets or chains is costly. Knowing why the failures occur, and taking measures to prevent them, could save the mine millions of dollars in unplanned downtime.

Having conducted a dozen Root Cause Analysis (RCA) inspections over a two year period of failed sprockets on armored face conveyors (AFCs), the following are my findings and suggestions as to what causes the failures.

As face lengths increased from the once-standard length of 600-700 ft, the horsepower (hp) had to follow with three times the 1,000 hp used on 1,000 ft face and three times the 1,900 hp used on the 1,500-1,750 ft faces. Today’s average length is between 1,000-1,500 ft, with one exception of a 1,750 ft long that I was directly involved with calculating fluid distribution from the high pressure pumps.

A previous article in Coal Age discussed a 2,000 ft longwall. It was stated that various operating, mechanical, hydraulic and equipment challenges would have to be overcome. The authors should have also included the possibility of poor face conditions that could result in a face this long.  Fluid distribution would be part of these problems too.

Some causes of sprocket failures include:

Lubrication and load;

Worn chain sprocket teeth;

Bearing failures;

Chain tension; and

Conveyor snakes.

Although not a common failure mode, lubrication and load fall into this list of failures, including contaminated grease caused by failed seals, the lack of lubrication, and the loading of the conveyor. This type of start up of a loaded face conveyor happens frequently. When this happens, it is not about start up of the loaded or overloaded AFC. The drive system needs enough ramp up time  to allow the full torque through the gearbox to turn the chain sprockets moving the conveyor.

The tailgate drive starts first, followed by the first and second headgate drives that kick in to complete the sequence of start. Fluid couplings and controlled soft-start (CST) drives are used.

It is important to note the position of the tailgate and headgate drives relative to the face conveyor when starting the AFC. The load and axial load transferred to the sprockets by the chain is of concern.

The tapered bearings below are a pair and face each other on one side of the sprocket. The fresh greased bearing, on the left, had no load contact, while the right, or partner bearing, had load contact.

The bearing failed because of incorrect preload and axial forces generated by the face chain. In all the failures I have examined, lubrication has not been the cause of failure.

This is considered normal. Ultimately snaking and chain tension, or lack of, are the probable cause of failure. In some cases the sprocket may be reversed, or switched, to use the good side. This can be scheduled, causing no loss of production, and may be done if the panel is near the end. There are always pros and cons to doing this. What caused the wear in the first place? Too tight a push at the gate ends? The wrong tension value, causing slack or too tight a chain? What is considered normal wear?

In my experience, bearing failures result from:

Lack of lubrication or contamination due to failed seals;

Overload or undue loading from another source;

Poor assembly;

Inferior design; and

Incorrect chain tension.

It is my opinion that axial loading, as suggested by the drawing, caused the failures and the incorrect bearing preload.

If tapered bearings are used, such as double spherical roller bearings, they do not take axial loading very well and will eventually fail. Double spherical roller bearings are self aligning and allow for some misalignment, not axial loading.

One inspection revealed a split inner race of the bearing, caused by being too tight a fit to the shaft.

Tension that is too tight causes high axial loading on the bearings when snaking. It can also contribute to an iron bound conveyor that will not snake correctly and load up at all connecting joints.

A tension that it too loose contributes to damaged sprockets and may result in broken chain.

Chain tension is the one factor that will ultimately cause sprocket failure, whether too loose or too tight. Similarly, when the face ends are too far in front or behind the rest of the conveyor line, the sprocket will also fail. This puts constant axial load on the sprocket teeth. This load is transferred to the bearings. It is very critical to maintain the correct tension to the face chain, adhering to the manufacturers’ recommendations.

Tension is achieved by the use of a hydraulic jack, or jacks, that have an 8-in. diameter piston mounted in line with the conveyor at the tailgate drive, so the jack can extend the slide pan and drive frame back away from the face conveyor to maintain tension.

The tension force of the jack is recommended by the manufacturer. This force is generated by system pressure supplied to a pressure reducing valve (PRV) connected to the longwall ring main. Pressure of around 4,200 psi is reduced to 1,800-2,100 psi, to generate a force extending the tail drive frame assembly away from the line conveyor. If using an 8-in. diameter piston, the force generated is pressure x piston area. For example, 1,800 psi x 50.266 sq in = 90,478 lb force, 2,100 psi x 50.266 = 105,558.6 lb force, and using 2,500 psi, the force generated is 125,600 lb. The PRV is usually a D500 Valve, which has been the main type of reducing valve for this application, used for years in the industry.

A photograph of a tensioning cylinder is shown above with external external reed rod position indicator to give minimum to maximum stroke of the piston, maintaining tension.

This is one aspect of instruction that all mechanics need to understand how to set the proper tension, to prevent premature failures.

The first part of this is directly related to the snaking of the conveyor at the headgate and the tailgate along with the shape of the drives, relative to the AFC.

Snaking changes the loading on the sprockets, and this axial type of loading should be absorbed by the bearing in the sprocket. It can
be tapered or spherical. This loading is created by the conveyor horsepower and the length of snake when preparing for the next cut. A long snake creates less axial forces than the short snake, which increases the axial loading chain to sprocket forces that ultimately reduces the sprocket life, or creates failure after a short period of time.

Spherical roller bearings are self aligning and do not take axial loading very well. Tapered bearings on the other hand are designed to take the axial loading if the preload is set correct. The greased bearings photo on the opposite page indicates incorrect bearing preload settings.

The drawings below, of a tailgate snake, indicate the theory of axial forces on the drive sprocket, and this would apply to both ends of the face.

The drawing below shows the forces generated by a high-horsepower AFC. The chain wants to stay in a straight line and the constant tension of at least 100,000 lb force on the chain adds to the problem.

This theory is based on 12 RCA’s on failed sprockets and bearing indication of loading, conducted by the writer.

I also suggest, after assembling sprockets, that they are checked for end play/float in this fashion, indicated in the following drawing by lifting off the bench. It is also good to record the information based on the type of bearings used.

In conclusion, chain tension values have to be maintained and checked on a regular basis as part of planned maintenance. Short snakes can cause undue axial loading on the sprocket bearings with the pre- set chain tension values. Running faces with the drives too far in front or behind the face will have constant loading on the sprockets, and can cause failure.

I also believe that short faces of 600 ft can be more prone to the axial forces described, although I have no data to confirm this.

There is one other important point: When installing a conveyor on a new longwall, and installing the chain, do not use all of the gearboxes built in chain tensioners to connect the chain. This will cause the AFC to become iron bound, and major problems will follow.

As technology continues to change, the AFC and chain sizes may increase based on the length. Also, the horsepower and expected tonnages for modern longwalls may increase. This also includes the type of drives used to move the chain sprockets. VFDs, fluid couplings and CST drives fall into this category.

Alcwyn “Al” Elliott retired after 45 years of working with the hydraulic and mechanical aspects of longwalls. During 2025, he published History of Longwall Coal Mining, which is available on Amazon. Al can be reached at: consult.my.path@gmail.com.