GB2128345A - Weigh frame - Google Patents

Abstract

A weigh frame for a belt conveyor has an idler roller 2 supported on a support member by a pair of idler support brackets 3. Load on the belt 12 is transmitted by the idler roller 2 and the support brackets 3, and is detected by strain sensing means 13 mounted on the support member t to measure its deflection. The support member 1 is mounted on the frame of the conveyor in such a way, e.g. by pivot joints 5, 6, 7, 8, that forces transmitted from the frame to the support member 1 are perpendicular to the support member 1. The relatively simple "bolt-on" weigh frame has improved accuracy as distortions caused by stresses in the conveyor frame are reduced. <IMAGE>

Description

SPECIFICATION Weigh frame The present invention relates to weigh frames for belt conveyor systems, and in particular to a mounting arrangement for a weigh frame in which stresses developed in the frame of the belt conveyor system are isolated from the sensing elements of the weigh frame, thereby reducing weighing errors caused by such stresses.

Prior art weigh frames are of several types, the first of which comprises a frame having two or more idler rollers which form a part of the system of rollers supporting the conveyor belt. The weigh frame is mounted via load cells located at a plurality of points such that the total load on the weigh frame is obtained by averaging the signals obtained from the plurality of load cells. Such a weigh frame forms an integral part of the conveyor system with which it is associated and must be incorporated into the system at the design stage.

A similar prior art weigh frame to the one mentioned above has a frame, one end of which is pivotally mounted at two points,and the other end of which is mounted at either one or two points, a load cell being provided at each of the latter nonpivoting mounting points.

It is also known to provide a burden indicator by attaching strain sensing devices to an idler frame in a belt conveyor system, such that the load on an idler may be determined by measuring the stress in the idler frame to which it is mounted. This arrangement has a significant cost advantage over the weigh frames previously mentioned, and is suitable for use in situations where a high degree of accuracy is not required. A burden indicator of this type is capable of error rates of below +5% and is suitable for use as an overload warning sensor among other uses and may be added to an existing installation with little difficulty.

One of the major factors limiting the attainable accuracy, in the simple burden indicator described above, is the existence of varying stresses in the conveyor system frame. These stresses are set up by various conditions acting on the conveyor system, such as temperature effects and wind loads which cause differential expansion and distortion within the structure.

A preferred embodiment of the weigh frame of the present invention provides a relatively low cost unit having a compact structure which may be fitted to an existing conveyor system with little difficulty, and in which the effects of stresses set up in the conveyor system structure are reduced, thereby allowing a higher accuracy to be attained than was possible with the previously mentioned low cost prior art system.

It is also possible to integrate a tacho-generator into the weigh frame of the present invention, thereby providing a complete mass flow measurement unit.

The present invention consists in a weigh frame for a belt conveyor system, said frame comprising: a substantially horizontal support member; a pair of idler support brackets mounted in spaced apart relation along the longitudinal axis of said member; an idler roller rotatably mounted between said brackets, the axis of said idler being substantially parallel to said longitudinal axis; and strain sensing means mounted on the support member and adapted to measure deflection of the support member, wherein each end of the support member is adapted to be mounted to one of opposite sides of a frame of the belt conveyor system, such that forces transmitted to said support member from said frame are directed substantially longitudinally of the support member.

In one form of the invention, one end of the support member is pivotably mounted at a single point, the pivotal axis of said mounting point being coincident with said longitudinal axis, and the other end of said support member is pivotably mounted at two points spaced on either side of said longitudinal axis, and the two mounting points have a common horizontal pivotal axis extending substantially perpendicularly to said longitudinal axis, said common axis intersecting with, or crossing in closely spaced relation to, said longitudinal axis.

In a preferred form of the invention the pivotal mounting points are provided by three clevis yoke and pin joints the axes of which all lay in the same horizontal plane as said longitudinal axis.

In another form of the invention one end of the support member is provided with a ball joint and the other end is supported by a flexure plate, the at least two mounting points being points along the continuous connection between the support member and the flexure plate and the pivotal axis laying in the plane of the plate.

In still another form of the invention the support member is suspended by three flexible wire ropes two of which are positioned at one end of the support member at spaced locations disposed one on either side of the longitudinal axis of the support member, the third flexible wire rope being positioned at the other end of the support member.

In the simplest form of the invention the weigh frame includes only one idler roller located substantially centrally of the support member, however, in some applications wing rollers are mounted on either side of the centrally located idler roller to provide a set of idlers which conform to the profile of the conveyor belt. The wing rollers are located with their axes in the same vertical plane as the idler roller and are mounted to one or the other of the idler support brackets, such that all loading from the belt is transmitted to the support member via the idler support brackets.

The number of pairs of wing rollers provided in a particular embodiment of the invention is dependent on the size and profile of the conveyor belt used in the particular application.

The weigh frame of the present invention may also incorporate a tacho generator for sensing idler roller speed in order to calculate mass flow rate and cumulative mass flow.

The support member is preferably an angle section or a tubular square section with the diagonals of the section placed in the vertical and horizontal planes respectively, or alternatively of tubular round section, however, other sections such as solid sections and channels, may also be used.

The weigh frame may inciude any number of strain sensing devices and in some applications one sensing device may be sufficient, however, in one embodiment, four strain sensing devices are provided in the centre of the support member each of the sensing devices occupying one face of the tubular square section, or in the case of a round section, each of the devices being centred on one of the four intersections of the tube wall with two radially extending planes which meet the horizontal at an angle of 450. The outputs of the sensing devices are averaged to provide a measurement of bending moment, with the two upper sensors being connected with opposite polarity to that of the two lower sensors, such that compression of the upper surfaces and tension in the lower surfaces, when the support member is under load, results in a signal of the same polarity from each sensor. With this sensor arrangement compression/tension stresses may be cancelled by averaging the outputs of each sensor, although, it will be recognised that cancelling of compression/tension stresses may also be achieved when any two sensors are used. The sensors are adapted to detect both compression and tension, and compression and tension forces of equal magnitude produce outputs of equal magnitude but of opposite polarity.

In some applications the preferred centrally located mounting of the strain sensing devices is not capable of producing the desired results, in which case other mounting techniques must be used, such as shear mounting wherein the sensing device is oriented at an angle of 450 to the normal orientation along the support member and is located away from the centre of the support member, such that the stress measurement is maximized. With shear mounting, torsional stress can affect measurements, however, with correct section of orientation and polarity of a pair of sensors torsional effects may be cancelled.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates in elevation a first embodiment of a weigh frame of the present invention, wherein the weigh frame is provided with only one idler roller; Figure 2 illustrates in plan, the weigh frame of Figure 1 in plan view; Figure 3 illustrates the weigh frame of Figure 1 viewed through sectional plane A-A; Figure 4 schematicaily illustrates the equivalent electrical circuit for strain sensors used in the weigh frame of Figure 1; Figure 5 illustrates in elevation a second embodiment of the present invention wherein the weigh frame is provided with a central idler roller and two wing rollers; Figure 6 illustrates an end elevation of the weigh frame of Figure 5 when viewed from the left;; Figure 7 illustrates an end elevation of the weigh frame of Figure 5 when viewed from the right; and Figure 8 illustrates a sectional elevation of the weigh frame of Figure 5 when viewed through section plane B-B.

Referring now to Figures 1, 2 and 3, the transverse support member 1, of the weigh frame, supports an idler roller 2 via a pair of idler brackets 3. The transverse support member 1 is suspended from a pair of mounting brackets 4 by a pair of clevis yokes 5 and pins 6 at one end and by a single clevis yoke 7 and pin 8 at the other end.

Each of the clevis yokes 5 and 7 have a jack bolt end fitted with two nuts to provide vertical adjustment of the transverse support member 1.

The axis of the pins 6 and 8 all lay in the same horizontal plane as the longitudinal axis of the transverse support member 1 with the axis of pins 6 perpendicular to the axis of the transverse member 1 and the axis of pin 8 coincident with the axis of the transverse member. This arrangement ensures that any forces transmitted to the transverse support member via the mounting brackets 4, clevis yokes 5 and 7 and pins 6 and 8 are applied to the transverse member 1 in the longitudinal direction, thereby avoiding the setting up of torsional forces or bending moments in the transverse member.

In use the weigh frame is mounted to the conveyor structure by bolting or clamping the mounting brackets 4 to opposite ones of a pair of parallel longitudinal stringers 11 of the conveyor frame. The height of the transverse member 1 is then adjusted using the jack bolts 9 and associated nuts 10 to bring the idler roller into supporting contact with the bottom of conveyor belt 12.

The load on belt 12 is determined by measuring the deflection caused in the transverse member 1.

This deflection is measured using four strain sensors 13 one located on each face of the transverse member 1 which has a square tubular section as seen in Figure 3. Each of the strain sensors is capable of measuring in compression or tension and when in compression give opposite polarity output signals to when in tension.

Therefore, by inverting the signals obtained from the two upper strain sensors, which are in compression when under load, with respect to the signals obtained from the two lower strain sensors which are in tension when under load, and averaging the four resultant signals, a signal related to deflection in the transverse member 1 is obtained. It will be noted that with this differential sensor arrangement any compression or tension forces applied to the transverse member 1 via its mountings will cancel, and thus not affect the weight measurement obtained.

Idler brackets 3 include provision for static calibration wherein calibration bars (not shown) may be placed between the brackets 4 and supported on edges 20 with retaining rod 21 preventing the calibration bars from sliding out of position. With this arrangement the stress applied to the transverse member 1 under the static load of the calibration bars is applied at the same point of the transverse member as stress applied under normal working conditions.

An equivalent circuit for the strain sensor is shown in Figure 4 where Figure 4(a) represents the unstressed sensor, Figure 4(b) represents the sensor in compression and Figure 4(c) represents the sensor in tension. It will be recognised by persons skilled in the art that by reversing the supply connections to a sensor of this type, the polarity of the output reverses for the same applied force, and it is this technique which is used to invert the outputs of the upper pair of sensors with respect to the lower pair of sensors.

Referring again to Figure 1, the sensors 13 are connected via cables 1 5 to remote electronics of known design. When used on outdoor installation a layer of thermal insulation 14 is also provided around the strain sensor elements. In installations in which higher accuracy is required, the transverse member 1 and the sensor elements 1 3 may also be maintained at a relatively constant temperature, despite variations in the ambient temperature, by circulating a fluid, the temperature of which is maintained at a relatively constant value, through the transverse member 1.

In order to determine mass flow and flow rate, it is also necessary to know the belt speed of the conveyor system. This variable can be measured separately, however, it is convenient to provide belt speed measurement as part of the weigh bridge unit, as this allows simplified installation wherein the complete measurement system is mounted as a single unit.

As seen in Figures 1,2 and 3 belt speed measurement may be achieved by providing a number of metal projections 1 6 on one end of idler roller 2, and positioning a reluctance sensor 17 in close proximity to the path of travel of these projections. The reluctance sensor 1 7 will then produce a series of pulses, each pulse corresponding to the passage of one projection 1 6 past the reluctance sensor 1 7. The frequency of pulses produced at the reluctance sensor output is proportional to the speed of the belt. A protective shroud 1 9 is provided for the reluctance sensor to prevent damage due to falling objects.

The output of the reluctance sensor 17 is connected via cable 1 8 to remote electronics where the signal is conditioned and combined with the signals from the strain sensors in a known way to produce the desired results.

Referring now to Figures 5-8, a second and preferred embodiment of the invention has a support member 31 to which are attached idler roller mounting brackets 32, 33, 34, 35, 36, and 37, the idler rollers 38, 39 and 40 being rotatably mounted on said mounting brackets.

The support member 31 is supported from a pair of parallel longitudinal stringers 42, 43 of the conveyor frame by three clevis yokes 44, 45 and 46 mounted directly to the stringers 42, 43 with the pins of the clevis yokes oriented in the longitudinal direction of the conveyor belt, there being sufficient play in the pin of clevis yoke 46 to prevent the occurrence of torsional stresses in the support member 31 due to the movement of stringers 42 and 43 with respect to one another.

Although the axes of the three clevis pins are not in the same horizontal plane as the support member 31, the distance "X" is kept to a minimum value which is determined by the relative heights of the base of the belt 47 and the top of the stringers 42 and 43, the distance between the top of the roller 38 and the support member 31 and the dimensions of the clevis yoke itself. In determining the minimum value for the distance "X", a provision must also be made to allow leveling and height adjustment of the support member 31 by way of the nuts on jack bolts 48, 49, 50. By keeping the distance "X" to a minimum, the torsional effects of end loading of the support member 31 will also be minimized.

When end loading is still a problem the hole in the ear 51 through which the pin of clevis yoke 46 passes may be extended or slotted horizontally to relieve any end loading.

The load on the belt 47 is determined by measuring the deflection in support member 31, using strain sensors 52 and 53 mounted on the bottom surfaces of the support member 31, however, if a four sensor system is required, as described for the first embodiment, additional sensors may also be provided on the top surface of the support member 31. Thermal insulation 54 is provided around the strain sensors 52, 53 and the sensor assembly is protected by an outer cover 55 which is in two parts connected by screws.

Calibration of the weigh frame is achieved in a similar manner to that of the previous embodiment, by placing calibration bars 56 on the calibration beam 57 and adjusting the gain and offset of amplifiers associated with the strain sensors to produce the desired result. The measurement of belt speed is also achieved in a similar manner to that used in the previous embodiment, a magnetic reluctance sensor 58 being positioned in close proximity to an end of the idler roller 39 and a sprocket wheel 59 being fixed to the adjacent end of the idler roller, such that when the roller 39 spins, the teeth of the sprocket wheel travel past the reluctance sensor 58 causing the sensor to produce a signal pulse for each sprocket tooth. These pulses may then be counted or integrated over a period of time to obtain a speed indicating signal.

The second embodiment of the invention is preferred for its ease of construction. This embodiment is produced by taking a standard idler roller set comprising the support member 31 the mounting brackets 32, 33, 34, 35, 36 and 37 and the rollers 38, 39 and 40. The remaining components are then simply added to the standard roller set.

It will be recognised by persons skilled in the art that numerous variations and modifications may be made to the invention as described above without departing from the spirit or scope of the invention as broadly described.

Claims (12)

1. A weigh frame for a belt conveyor system, said frame comprising: a substantially horizontal support member; a pair of idler support brackets mounted in spaced apart relation along the longitudinal axis of said member; an idler roller rotatably mounted between said brackets, the axis of said idler being substantially parallel to said longitudinal axis; and strain sensing means mounted on the support member and adapted to measure deflection of the support member, wherein each end of the support member is adapted to be mounted to one of opposite sides of a frame of the belt conveyor system, such that forces transmitted to said support member from said frame are directed substantially longitudinally of the support member.

2. The weight frame as claimed in claim 1, wherein first mounting means provided at one end of the support member is adapted to allow said one end to be pivotally mounted at a single point and second mounting means provided at the other end of the support member is adapted to allow said other end to be pivotally mounted at two points, said single point being disposed substantially in a vertical plane passing through the support member and said two points being spaced either side of said vertical plane.

3. The weigh frame as claimed in claim 2, wherein the mounting points are all disposed substantially in a horizontal plane passing through the support member.

4. The weigh frame as claimed in any one of the preceding claims wherein the strain sensing means are provided in the centre of the support member.

5. The weigh frame as claimed in any one of the preceding claims, wherein said strain sensing means comprises a plurality of strain sensing elements arranged to allow cancellation of the effects of unwanted stresses in the support member.

6. The weigh frame as claimed in any one of the preceding claims wherein the strain sensing means are mounted in sheer.

7. The weigh frame as claimed in any one of the preceding claims wherein means are provided to keep the support member at a substantially constant temperature.

8. The weigh frame as claimed in any one of the preceding claims, wherein calibration means are provided to allow the positioning of calibration weights on said weigh frame such that the effect of said weights is substantially the same as that of an equivalent weight bearing on said idler rollers.

9. The weigh frame as claimed in claim 8 wherein the calibration means consist in a pair of beams attached to and extending transversely of the support member, the beams being equally spaced either side of the sensing means.

10. The weigh frame as claimed in any one of the preceding claims wherein a plurality of idler rollers are provided along said support member.

11. The weigh frame as claimed in any one of the preceding claims, wherein speed sensing means are provided to sense the rotational speed of the idler roller or at least one of the plurality of rollers.

12. The weigh frame substantially as hereinbefore described with reference to Figures 1-4 or Figures 5-8 of the accompanying drawings.