How to calculate the maximum operating tension of conveyor belt?

Maximum operating tension is generally characterized in terms of pounds per inch of width and should be matched as closely as possible to the PIW working strength of the belt constructions under consideration. The working strengths of specific SKE conveyor belt constructions can be found in the appropriate heavy duty belt brochure or belt specification data sheets.

Maximum operating tension, the highest tension occurring in any portion of the belt on the conveyor system under operating conditions, is a prime consideration in selecting the right belt. The conveyor system applies an appropriate amount of power to the belt in order to drive the belt at design speed. This power must be sufficient to accelerate and drive the empty conveyor, to move the material horizontally and vertically, all within the design of the conveyor system, and to overcome all flexural, inertial, frictional and gravitational forces operating on the system.

These aforementioned forces create tension in the belt. The amount of tension created can be computed in the time-honored fashion by careful consideration of each of these forces; however, there is a “Quick Method" which can be used and which generally proves satisfactory. Initially, let us consider effective tension.

Effective tension (Te) is the tension created in the belt when sufficient power is applied to the system to drive the conveyor belt at a desired speed. This relationship can be derived from a knowledge of motor horsepower and belt speed as follows:

Te = (Hp x33,000)/belt _ speed (belt speed in feet per minute)

Belt conveyors utilize a friction drive and accordingly, when power is applied to the drive system, one run of the belt will experience a higher tension than the other.

Let us call this the tight side tension (T1) and the other run, the slack side tension (T2) Upon installation, a belt is normally tensioned until the belt fails to slip with the system fully loaded. The amount of slack side tension required to prevent slippage at the drive is a function of several constant factors:

1. The coefficient of friction between the drive system and the belt (whether conveyor pulley is lagged or not)
2. Belt wrap at the drive, and type of drive. Type of drive is important, since this has a direct bearing on how the motor applies driving force to the belt. This has a direct impact on the maximum tension to which the belt will be exposed. (See illustrations on the following page).
3. Type of take-up (whether screw or gravity).

The ratio of slack side tension (T2) and effective tension (Te) can be represented by a constant.

K = T2 /Te; Therefore; T2 = KTe.

Maximum operating tension (tight side tension) can now be computed by T1=Te + T2 times the starting factor (1.5, 2.0, 3.0 etc.). Electric motors may have very high starting torques. CEMA recommends the use of a starting factor (multiplier) to compensate in calculations.

In the quick method we equate maximum operating tension to tight side tension since we are using a generous safety factor - total motor horsepower. For conventional conveyors designed with a gravity take-up located near and behind the drive area, the counterweight (Cwt) has a direct relationship to the slack side tension (T2). The amount of Cwt can be expressed as:

Cwt = 2T2

This Cwt is the total amount of counterweight needed in the system to maintain proper conveyor tensions. It should be noted that this total also includes the weight of the take-up pulley and the take-up weight box and appropriate hardware associated with the gravity take-up. As the gravity take-up location moves toward the tail area and further away from the drive, the amount of take-up weight will increase. This increased take-up weight will be needed to overcome the return run friction factors between the drive and the take-up location.

The four drive systems shown below are only four of the many drive arrangements that can be constructed depending on the general conveyor profile.

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