Courtesy : Trans Tech Publications - Bulk Solids Handling Journal

Summary

common problem arising in the open pit industry is the determination of motor torque required to drive a belt conveyor when belt length increase is proposed, belt lift is changed, or tonnage increases are sought. Strain gauges attached to the drive shafts of the conveyor may be unreliable for many reasons and therefore reliable alternatives have been developed and tested that give in-situ values of running torque and conveyor friction. Three alternative non-strain gauge methods are available that can be used as a cross-check against each other, including belt vibration to tension analysis, belt sag distribution to tension and dynamic direct belt tension measurement of T₁ , T₃ and T₂ values. Additional valuable information on belt quality (loadsharing of stress) and take-up dynamics is a by-product of these testing procedures. Permanently installed tension monitors will indicate increases in torque requirement due to conveyor component wear, seizure, or gradual misalignment. Application to design upgrading is discussed in this paper.

1. Introduction

Conveyor design has evolved to the point where belt tension computations are readily modelled by a variety of calculation methods such as ISO 5048, CEMA and DIN 22101. A particularly noteworthy trend in modern, high capacity mines is the need to upgrade conveyors to higher capacities, longer lengths or to change the lift; characteristics as the mine terrain advances.

There are technical considerations related to the continually varying mine conveyor, including:

1. belt safety factor characteristics [1]

2. belt dynamic response to starting or stopping [2]

3. belt quality and condition-joining new belt to old belt [3]

4. drive torque capability evaluation.

The last subject involves quantifying the existing conveyor in terms of drive power and torque requirement particularly if there are non-linear torque losses at the drive coupling interface that result in drive speed slip on starting or running.

Three distinct methods are available to determine the torque levels required by existing conveyors in relation to modelled torques, namely:

1. Direct strain gauging of the drive shafts

2. Direct measurement of belt tension by load cells

3. Indirect measurement of belt tensions T₁ and T₂.

In this paper, a brief overview of methods 1 and 2 will be presented. The method of indirect tension determination (method 3), however, is of greater interest due to its accuracy and practical implementation on site.

2. Conveyor Torque Problem

When a conveyor is required to carry material over longer distances by increasing lift or by extending the belt, the question of torque required becomes an important factor due to the possible need to install new motors. The installation of new motors may be critical to the operation of the modified conveyor. Motors installed at an incorrect location of the new profile, or motors that have insufficient torque, could result in an inability to start the belt.

Similar comments apply to underground coal mining belts where the undulations of the seam, together with extending or retreating paneis, may result in a conveyor that has inadequate belt tension rating, carrying capacity or drive torque capacity. Measurement of conveyor torque (M_c) of an existing conveyor with a single equivalent drive drum (radius r) is governed by the equation

M_c = f₁ (r, T₁, T₂, Q)         (1)

        = T_e. r = (T₁ - T₂) . r      (2)

where T₁ and T₂ are the tight and slack side tensions and the temperature Q is in C. The function f₁ is a multiparameter by nature.

The motor torque M is not necessarily transferred to the drive drum with linearity during start-up if there are viscous couplings in the circuit, and so the motor torque required to move the conveyor may be a complex function of speed v

which involves a dimensioniess function f₂. For a locked coupling f₂ = 1, approximately.

Measurement of M_c by a variety of methods will permit a reasonable understanding of the operation of the present system. Extension to longer systems requires modelling of the changes in T₁ and T₂ tension followed by comparison of "old" conveyor torque (measured) to "new" conveyor torque (predicted). Comparison with motor currents ("old") with what will be expected ("new") can also be made. A basic equation for determining the "new" torque from present torque measurements is

M_c(required) = (M_c/r + ΔT_e) . r       (4)

where ΔT_e is the change in the effective tension due to the proposed modification. Measurement of M_c and prediction of the additional T_e is therefore required to determine future torque requirements from the drive. A compound drive (primary, secondary or separated drive) requires torque to be split between drives.

3. Review of Methods

3.1 Strain Gauging

Strain gauging and telemetry methods require accurate application of strain sensors to ensure proper bonding to the steel shaft, proper orientation to ensure bending stress is removed from the torsional strain measurement, proper location of the gauge in relation to shaft diameter changes, accessibility and multichannel telemetry. Microstrain variations need to be calibrated accurately, and the steel modulus is required to calculate the stress, at shaft. The torque needs to be computed based on assumptions about the actual radial arm from the belt line to the strain gauge. Zones of influence also need to be taken into account well in advance by preferably Finite Element modelling of the end-disc/ shaft to determine zones for placing the gauge in a uniformly strained region.

3.2 Load Cell Application

Load cells need to be installed under idler supports or under pillow blocks of bend pulleys to monitor the T₁ and T₂ tension loads. Monitoring T₁ and T₂ of an existing conveyor requires

1. calibration of the load cells

2. installation in high tension areas

3. vector analysis of the belt forces around bend pulleys or over instru mented idlers.

There are practical considerations and limitations in the installation of load cells that are overcome by method 3 if on-site data is required from any conveyor by a general method.

3.3 Indirect T₁ and T₂ Measurement Methods

Considerable success has been achieved with the use of natural "belt quality" measurements of frequency and sag. Measurement of frequency of a belt under tension gives a direct ability to compute the belt tension using plate theory methods [3]. The essential processes used in the measurement of T₁ and T₂ tension based on frequency include

1. installation of non-contact displacement sensors at T₁ and T₂

2. excitation of the natural modes of transverse plate vibration of the belt

3. collection of the plate-mode frequencies

4. computation of the belt tension T₁ and T₂.

These methods can be easily cross-checked by other means without permanently installing hardware such as is the case in section 3.2. The tension of a belt, based on a measured modal frequency is

f = 1/2π (√af₃ + bf₄)          (5)

where a is the belt stiffness/mass ratio, f₃ is a non-dimensioniess frequency parameter that depends on the shape of the measured mode, b is the tension (T) to mass (m) ratio, and f₄ is a modal constant [3]. When b = T/m, the value of T can be determined (T₁ or T₂ depending on 1ocation of test).

The value of T₁ or T₂ can be readily cross-checked by a sag measurement process in which belt tension is given by a function of sag and belt mass "m" [4],

T = f₅ (sag, m)        (6)

Both frequency and sag methods lend themselves to easy setup in the field and to internal cross-checking of conveyor torques.

Accuracy of the methods has been established over many years and typically a T₁ or T₂ value of 5% error is obtainable with a good measurement setup.

3.4 T₂ Belt Tension Measurement

In many conveyors, the determination of T₂ belt tension is facilitated by an ability to monitor takeup rope loads, by the vibration method applied to ropes. This gives an ability to measure T₂ and cross-check its value in three ways, namely, by rope frequency-to-tension, belt frequency-to- tension and sag-to-tension measurements.

In addition to being able to monitor tensions in a rope by the vibration method, a number of rope falls in a typical takeup system may be tested, from which

1. takeup mass is determined directly

2. takeup friction can be determined in relation to dynamic sag measurements on start or stop

3. real T₂ belt value is determined irrespective of rope/sheave friction losses.

The ability to determine T₂ accurately has considerable value when predicting a system's performance requirements, since the takeup tension value forms the basis for setting the high tension in the beit and the effective tension around the drive drum.

4. Permanent Installation Applications

Advantages of these simple methods of monitoring belt T₁ and T₂ tension include longer term sensing of drift in installation friction due to shiftable structures and wear of idlers. Additional friction in the takeup that could predispose the system to slip can be monitored as can the effect of grade changes and tonnage variations on motor torque requirement. Permanently installed monitors can be periodically interrogated to monitor changes in operating torques in situations where motor ratings are close to maximum limits of motor operation.

5. Concluding Remarks

In this brief paper, methods have been described that are used to monitor belt T₁ and T₂ tension directly, from which conveyor torque values are determined for future upgrades. There are considerable practical advantages when using frequency and sag methods to determine belt tension, and the fact that permanent monitoring is available is a big advantage to operators that have shiftable conveyors with continually changing T₁ tension. Permanent installation devices also have an ability to monitor the T₂ belt tension, and aid PLC controllers in monitoring conveyor slip potentials as takeups become stuck or increase their friction levels.

References

1. HARRISON, A.: Safety factor calculation for inclined high-strength belts based on NDT signal analysis; Coal Handling & Utilisation Conference, Sydney, Australia, 19-21 June 1990, pp. 289- 295.

2. BARFOOT, G.: Computer modelling of belt conveyor systems; National Conf. on Bulk Materials Handling, IEAust., Australia, 1993, pp. 151-157.

3. HARRISON, A.: Determination of the natural frequency of transverse vibration of conveyor belts with orthotropic properties; JS. & V., Vol. 110 (1986) No. 3, pp. 483-493.

4. Harrison, A.: New concepts for evaluating belt tracking and dynamic tensions; Mech. Eng. Trans., IEAust., 1989, pp. 114-118.