Anyone relying on conveyor belting to transport material understands the negative results of unexpected or premature belt failure. Realizing that belt lines are the lifelines of any bulk material handling operation, it is easily accepted that proper care and extra time are necessary to help prevent the high cost of unexpected downtime as a result of misapplication, inadequate maintenance or improper installation of the conveyor belt.

Unfortunately, it is not possible to guarantee that a conveyor system will not create unplanned outages, but there are some things that can be done to keep such problems to a minimum.

Although there are several areas that deserve attention, priority should be given to the following three areas:

* selecting the proper conveyor belt specification for each system;

* implementing a well-planned, ongoing conveyor belt maintenance program; and

* installing the conveyor belt correctly.

Taking short cuts in any one of these areas will result in a greater probability of downtime and a higher production cost.

Installing an inferior or improper conveyor belt construction on a system can definitely lead to problems. There are six basic considerations requiring great care when specifying a belt for each conveyor system.

Step 1: Belt Tension The first-and perhaps the most important-consideration is tension, that is, the force tending to elongate the belt. Tension is expressed in pounds per inch of width (piw). The three types of tension critical to providing a proper recommendation are:

* tension to overcome the friction of the conveyor components;

* tension to overcome the weight of the load; and

* tension needed to raise the load through any elevation.

In the majority of conveyor belt applications, the above three types of tension can be considered by simply using the "quick-check method" to determine the approximate maximum tension. This method should not be used as the sole recommendation for designing new conveyor systems, complex systems where trippers or unusual components are evident, or for belts carrying material on a decline.

To use the "quick-check method," ascertain the belt width and speed, the motor horsepower, if it has lagged or bare pulleys and the type of take-up on the system.

The following example problem shows how the "quick-check method" works. After inspecting the system, the following information was learned:

* Belt width - 36 in.

* Horsepower - 25 hp

* Belt speed - 350 fpm

* Type of pulley - lagged

* Type of take-up - counterweight

The belt's approximate tension can be determine by using Table 1 (see page 44). This example has the belt speed at 350 fpm. Find that figure on the left-hand column of the chart. There is a counterweight take-up and the pulley is lagged, so use the corresponding figure in the second column (117).

Multiply this figure by the motor horsepower (117 yen 25 equals 2,925). This is then divided by the belt width. That figure is approximately 81. On tension alone, a low-tension belt such as 250-piw construction would be acceptable for this application. For exact tension requirements, a conveyor belt supplier should conduct a tension calculation for each system throughout an operation.

The conveyor belt must have sufficient transverse rigidity (lateral stiffness) to support the weight of the material carried over the idler junction. If the load is too heavy, the belt will be forced between the idlers and will wear prematurely. To determine if the recommended belt has the transverse rigidity to support the load, one needs to know the weight of the material being conveyed, the degree of the idler trough, the width of the belt and the information in Table 2.

Step 2: Load Support Consider the example used for tension and assume the material is washed gravel that weighs approximately 100 lb per cu ft. Also assume the idler trough measured 35 degrees.

In Table 2, the middle sub-column of the 81-120 lb/cu ft column contains the 35 degrees idler figures. Cross-matching this column with the 250-piw specification shows that the maximum allowable belt width for this weight of material at this idler angle is 42 in. Therefore, the 250-piw belt does meet this requirement.

Step 3: Troughability The belt must be flexible enough to trough or make contact with the center roll idler while empty. While Table 2 showed the maximum allowable width for the belt, the troughability will tell the required minimum belt width.

The belt in figure A will not trough; it will not rest on the center roll. In this condition, the belt will wander and be difficult to train. The belt in Figure B is troughing and will properly steer.

Belt width and the troughing angle of the idlers is the only information needed to determine the belt's troughability.

Using the same example, it is known that the 250-piw construction will handle the belt tension and load support. It is also known that the troughing idlers are 35 degrees.

According to Table 3 (see page 46), the minimum belt width for the 250-piw belt is 18 in. Therefore, the 250-piw belt is still acceptable for this particular application. A 36-in. belt was used for this example. The belt carcass and covers must be able to withstand abnormal impact from certain materials and poor loading conditions. An extra ply, extra cover gauge or a different rubber compound is generally recommended for severe-impact service or where loading conditions dictate a unique belt requirement.

Step 4: Belt Impact Inspect the condition of the old belt to determine whether an application requires additional cover material or plies. Look for belt damage and cover separation. Determine if the belt has become unserviceable due to impact. Also, inspect the loading sites. Is material falling at an inappropriate angle, or falling too far or too fast?

Step 5: Flexible The quality and design of the belt's rubber and reinforcement must be sufficient to withstand the flexing associated with small-diameter pulleys. To determine whether a belt recommendation will be satisfactory for a specific system, measure the diameters of all the pulleys on the system. Use the tension figure obtained earlier. Finally, refer to the belt supplier's information on this subject. Table 4 is a typical chart for selecting the proper belt for system pulleys.

Determine the actual percentage of tension the belt will be subjected to compared to what it is capable of handling. A 250-piw belt is designed for a maximum of 250 piw. In the example, the tension figure determined by the quick-check method is 81 piw, which equals 32% of 250.

With that figure in mind, refer to Table 4. The minimum recommended pulley diameter for this application is 10 in.-the tail and snub pulley are also at 10 in.

As long as the pulleys are a minimum of 10 in., the 250-piw belt recommendation is acceptable. For the system that has pulleys less than 10 in., recommend larger pulleys, as inadequate pulley size can create downtime due to premature splice failure.

Step 6: Belt Covers The covers are the most obvious portions of the belt, and play a major role in belt life. In most belt constructions, there is a top cover and a bottom cover. Review each application to determine if heat, oil, abrasion or a combination of these conditions exist. Also, consider any special cover requirements, such as Mine Safety and Health Administration approval.

The top cover of a conveyor belt is there for only one reason-to protect the fabric, or strength member, from deterioration. The bottom cover also protects the fabric reinforcement from premature wear caused by the pulleys and idlers. Depending upon the application, the covers can vary in gauge, compound and configuration.

Determining the top cover gauge for a belt depends on the application and the system. The best way to determine what the needed gauge may be for a belt is to look at the old belt and see what cover gauge it had. If that belt offered the desired level of service, it would be wise to maintain the same specification.

If the top cover wore prematurely, consider a heavier gauge or a different compound. A life comparison should be made for each specification because, on average, 30% more rubber is received for only 10% more money. Conversely, if the top cover still had plenty of usable rubber when the belt became inoperable, a lighter cover might be better for that particular application.

Table 5 is intended strictly as a guide for top cover gauge selection. A precise table is impossible because final selections are heavily dependent upon experience, previous belt history and a general appraisal of the factors which follow. Final selections may be based on a single dominant factor or on a group of factors. Final gauge selection may sometimes be lighter or heavier than the indicated ranges for special reasons.

There are six factors favoring heavier gauges:

* low end of time cycle range;

* high end of lump size range;

* poor loading-history of cutting, gouging, wear, etc.;

* ultimate belt life desired;

* belt in constant usage; and

* costly carcass.

There are five factors favoring lighter gauges:

* high end of time cycle range;

* low end of lump size range;

* good loading-normal cutting, gouging, wear, etc.;

* belt of secondary importance-downtime not a serious factor; and

* belt in intermittent usage.

1. Make sure the belt has the necessary strength to handle the tension requirements.

2. Determine the minimum number of plies or carcass gauge to ensure the conveyor belt offers sufficient load-support characteristics.

3. Make sure the conveyor belt offers necessary troughability characteristics to make good contact with the center idler roll in light loading and empty conditions.

4. Select a belt with the impact resistance to handle the load as it falls onto the belt.

5. Select a belt with sufficient flexlife capabilities to effectively operate around the terminal pulleys.

6. Determine the proper covers to handle the material and loading conditions of a given system.

Martin Engineering plans to take its conveyor dust- and spillage-control workshops to 12 U.S. locations this year. The one-day workshops will cover the cause of fugitive material, ways to control air movement in conveyor loading zones, the importance of wear liners inside the chute, sizing dust collection systems, preventing leakage where the belt enters the loading zone, and causes and cures for belt wander. The workshop qualifies for 1.6 continuing education units.

The remaining dates and sites of the workshops are:

* April 21, Sacramento, Calif.

* May 1, Des Moines, Iowa

* May 5, Lexington, Ky.

* June 16, Lansing, Mich.

* July 21, Seattle, Wash.

* Sept. 15, Kansas City, Mo.

* Sept. 29, Albuquerque, N.M.

* Oct. 20, Dallas, Texas

* Nov. 10, Bakersfield, Calif.

* Dec. 1, Orlando, Fla.

For registration or information call, (800) 333-4294, Ext. 319. Registration forms are available at www.martin-eng.com