Monitoring tension in an elongate flexible member

Details Monitoring tension in an elongate flexible member Monitoring tension in an elongate flexible member

1988-10-11

1990-05-29

Ruehl, Charles A.

Measuring and testing

Dynamometers

Responsive to force

73DIG1, G01L 504, G01L 110

Patent

active

049285386

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to monitoring tension in an elongate flexible member, in particular tension in a flexible drive belt which passes around a plurality of pulleys.
In many types of industrial machinery, flexible drive belts are used to transmit power and/or to provide timing synchronisation. These belts may have flat surfaces or may have surfaces which are grooved or ribbed to increase their ability to transmit power without slippage.
Whatever their construction such belts, for correct operation, must usually be set to within a special range of tensions. A belt which is too tight may be likely to suffer excessive wear and deterioration in operation, while a belt which is too loose may slip laterally off a guide pulley, or slip longitudinally, or jump teeth in the case of a ribbed belt. Thus an essential part of the initial setting up of the machine may be setting the belt tension to within an acceptable range of values. Particular difficulties arise in measuring the tension in a drive belt which has been installed around pulleys, one of which is the limited space that may be available for measuring the tension.
Currently available devices for measuring drive belt tension are purely mechanical. These devices clamp on to a short section of the belt and predict tension either by applying a known force and measuring belt deflection, or by applying a known deflection and measuring force. Through tests carried out in the laboratory using a tensile testing machine we have shown that these devices give both poor accuracy and poor repeatability of results. Measuring errors of up to 60% can occur in a random manner. One of the reasons why error occurs is that, if the belt slips by even a small amount where it is clamped, this will significantly alter the force/deflection characteristic.
Thus there is a real need for an improved drive belt tension monitoring device and method capable of achieving better levels of accuracy and repeatability in monitoring, and where appropriate measuring, the tension of a flexible drive belt. It is an object of the present invention to meet that need.
According to the invention there is provided a method of monitoring the tension in a flexible drive belt which passes around a plurality of pulleys, the method including the steps of causing a portion of the belt when installed around the pulleys to vibrate at a natural frequency and detecting the frequency of the resulting vibrations.
The term "natural frequency" as used herein refers to any frequency at which the belt will naturally vibrate. In the normal case this will be the fundamental frequency but it is possible to arrange for the belt to vibrate in harmonic modes at multiples of the fundamental frequency of vibration.
We have found that the natural frequency at which the belt vibrates provides an accurate and reliable indication of the tension of the belt. Although such drive belts are of significant thickness and do not therefore vibrate exactly in the manner of a simple string, we have found surprisingly that the technique of monitoring the natural frequency of vibration of the drive belt does provide a reliable indication of the tension in the belt.
The ends of the portion of the belt which is vibrated may be defined by points of engagement of the belt with the pulleys. In this case there is no need to provide any additional clamping arrangement for fixing ends of the vibrating portion of the belt.
The belt may be vibrated by the application of an initial impulse thereto and thereafter be left to vibrate freely, substantially without the influence of external forces. In this case the belt automatically vibrates at a natural frequency.
Alternatively the portion of the belt caused to vibrate may be vibrated by a drive mechanism at a natural frequency and at a substantially steady amplitude while the natural frequency of the vibrations is detected. This method has the advantage that there is a steady vibration of the belt which facilitates measurement of the frequency of that vibration.
The drive mechanism may be driven

REFERENCES:
patent: 3470734 (1969-10-01), Agdur et al.
patent: 3719073 (1973-03-01), Mahon
patent: 3850031 (1974-11-01), Schwenzfeier et al.
patent: 3854329 (1974-12-01), Jones
patent: 3871217 (1975-03-01), Miley
patent: 4565099 (1986-01-01), Arnold

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