Using Hydraulics as a Scale on the Farm

Craig W. Yohn, Extension Agent, West Virginia University

Dr. Dana Porter, Extension Agriculture Engineering Specialist, Texas A & M University

Dr. Edward B. Rayburn, Extension Forage Specialist, West Virginia University

Scales are often used on the farm to weigh things such asfeed, lambs, calves, and square bales of hay. Larger objects suchas round bales, silage, or livestock may be weighed less often,if at all. In many cases, scales for this type of measurement maycost more than $1,000.

Hydraulic cylinders and front-end loaders can be used asaccurate scales for a fraction of the cost.

Basic Principle

"Hydraulics" is a term the industry commonly uses todescribe the science of transmitting force and/or motion throughconfined liquids. "Power hydraulics" and"hydrostatics" are more specific terms used in industryfor the field.

In hydraulics, pressure is required for pushing or exerting aforce or torque. In a hydraulic system, pressure controls force.

Pressure is defined as a force per unit of area or

For example, if a hydraulic system operates at 20 pounds persquare inch (psi) of pressure, the hydraulic fluid is under apressure of 20 psi.

Application -- StaticHydraulic Cylinders

The terms "force" and "weight" can beinterchangable. The previous formulas can be used to determinethe weight of an object based on the pressure exerted on ahydraulic system. A single hydraulic cylinder or severalcylinders connected in a series with an object hanging belowwould exert pressure on the system.

Figure 1 illustrates how a single cylinder could be configuredto measure the pressure exerted by the weight. A quick couplingsystem allows the same gauge to be used with severalapplications. Make sure that the cylinder is filled withhydraulic fluid before attaching the gauge.

Figure 1

Table 1 provides guidelines for usingdifferent sizes of cylinders for different weighing capacities.

* The area of a cylinder or rod is determined by multiplying the radius of the circle by itself and multiplying that value by 3.146(pi). (Area of a circle = Pr²).

** Area of the cylinder minus the area of the rod.

*** The suggested operational maximum weight is approximately 80% of the calculated maximum weight. It is subject to the proper calibration of the cylinder.

Calibration

The figures in Table 2 are based on calculations that assumetheoretical values. They provide guidance in the design of aweighing device. Only a couple of factors may justifycalibration. Not knowing the gauge is accurate and not knowingthe correct diameter of the cylinder or rod can drasticallyaffect the accuracy of the scale. In general, the theoreticalvalues are valid. To prove this, a 2-inch cylinder with a 1-inchshaft was tested using 1000 pounds of tractor weights which wereweighed on a certified scale. The cylinder has a theoreticalvalue of 2.36 pounds of weight per pound of pressure on thegauge. The test comprised ten replications, starting with theweight of the platform and adding 10 100-pound weights at a time.

Table 2

The calibration showed an average difference of only 0.01pounds of pressure per pound of weight from the theoretical valueof 2.36. This cylinder, used to 1,000 pounds, withoutcalibration, would give a reading 5 pounds lighter than theactual weight. Another way of evaluating the accuracy would be tosay the measurement is 99.6% of the actual weight.

More than one cylinder in a series or parallel circuit can beused to weigh larger amounts. A parallel connection will dividethe load between the cylinders. This would require a gauge oneach cylinder, but would allow for a capacity without increasingthe cylinder size. A series connection would need only one gauge,but all cylinders would have the same load, reducing the capacityof the scale in comparison to the same number of cylindersconnected in a parallel circuit. All gauges, fittings andhydraulic hoses must be rated for hydraulic use at no less thanthe rating of the cylinders used or the maximum pressure thatwould be reached by the scale.

Application - Front-end Loader

A hydraulic gauge also can be used to turn a hydraulicfront-end loader into a scale. There are differences incalibrating this system versus a static hydraulic cylinder. Afront-end loader uses many feet of hydraulic hose. It has ahydraulic pump and reservoir. Front-end loaders are alsodifferent in design from manufacturer to manufacturer and usedifferent attachments for lifting. The relationship of weight topressure will change for the same equipment if differentattachments are used. The pressure will be different for a bucketversus a fork, versus a hay spike, versus a bucket with a hayspike attached. The farther away the load extends beyond thecylinders or the hinge point, the greater the pressure must be tolift the same weight.

Calibrating the front-end loader will take only a few hours.Objects of known weight are lifted by the front-end loader andthe pressure is noted.

Procedure for Large Round Bales

1. On a certified scale, weigh at least three bales of various size, condition, or species makeup.
2. Lift each bale so that the loader is at the same height and record the pressure on the gauge.
3. Divide the weight of the bale by the pressure read on the gauge.
4. Calculate an average ratio.
5. The average ratio of weight to pressure is then used to calculate the weight of other bales.

For example:

Three bales of different sizes and makeup were weighed on alocal certified truck scale.

An average ratio is determined by adding the three readingstogether and dividing:

The average ratio is 1.663. A table can then be generated touse when weighing bales by multiplying pressure times theratio (ex. 300 * 1.663) and then rounding to a whole number.

Table 3 is an example based on the average ratio in theexample above.

Tractor weights, sacks of feed, or other objects of knownweight could be used to calibrate. The important thing toremember is to place the known weight in the same location on thefront-end loader as the commodity being calibrated.

A "quick coupling" system can be built with partsfrom many different suppliers for less than $100.

Figure 2 shows how such a system could be constructedto be a part of the existing hydraulics of the front-end loader.

Suppliers

There are many suppliers of cylinders, gauges, and hydraulicfittings and hoses. These include auto part stores, farm supplyand implement dealers, and specialty suppliers such as industrialhydraulic and pneumatic system installers.

When considering what to purchase, keep in mind the range ofweight that is being measured and the accuracy desired. Thelarger the cylinder the more weight capacity, but the largerincrement of weight per pound of pressure. For example a 2-inchcylinder, with a pressure capacity of 3000 psi, has the capacityto weigh up to approximately 5,700 lbs. Each pound of pressurewill equal approximately 2.36 pounds of weight. A 3-inch cylindermay have a capacity of over 10,000 pounds and each pound ofpressure would equal approximately 6.0 pounds. The same is truefor pressure gauges. An oil filled gauge is recommended.

A gauge with a 500 psi capacity may have 10 psi increments,and a gauge with a capacity of 1,000 psi may have 20 psiincrements.

A kit is available from Weigh-All Field Scale ofFort Gibson, Okla., for less than $250 if you would ratherpurchase than build and calibrate your own system.

Summary

Hydraulics can be a powerful tool to record the agronomicalperformance of the farm and provide information related tofeeding animals. The construction, configuration, and calibrationof the scale are well within the capabilities of most producers.

Calibration is a must for different temperatures,apparatus, and age and wear of the cylinder or hydraulic systembeing used as a scale.

Parts are available through many companies. The pressure gaugecapacity and the hydraulic hose and fittings should all meetstandards greater than the maximum pressure of the hydrauliccylinder, the operating pressure of the tractor, or the maximumpressure generated by the weight to be measured.

Acknowledgments

The authors wish to thank the following individuals and farms:

Edmond B. Collins, Associate Director, WVU Center for Agricultural and Natural Resources Development

Michael J. Kridle, WVU Extension Graphic Arts Designer

Meadow Green Farm

Windy Knoll Farm

Lyle C. Tabb & Sons