How is Hydraulic Press Capacity Calculated?

Hydraulic presses can apply large amounts of force to mold metal parts. The amount required depends on both the material being formed and its energy requirements for shaping it into desired forms.

Press capacity is typically expressed as the depth of blank (DOB). But what happens if the workpiece needs to be formed beyond this DOB?

Force

Hydraulic presses generate force via the ratio between piston area and cylinder area. Assuming a large piston is moving against a smaller cylinder, pressure exerted upon it is proportional to their total surface area; larger cylinders support more weight.

Calculating press capacities requires taking elastic deflections into account, including both forward "snap-through" and reverse tonnage - depending on material thickness and strength as well as press speed.

Calculating energy requirements can also be done using an easy formula: Force = Pressure x Diameter. In general, higher pressures and larger cylinders and beds tend to generate more force - however when selecting a hydraulic press it's essential that ram stroke length and cycle time be considered when choosing the best hydraulic press for you application.

Cylinder Area

Hydraulic presses can generate enormous amounts of energy thanks to Pascal's Law, which states that any pressure exerted upon a fluid will be transmitted along all points touching it.

This principle allows relatively small cylinders to move incredibly heavy loads. To estimate the force a cylinder can generate, one needs to know its surface area and pressure rating - F=PA provides the optimal way of doing this calculation.

Hydraulic presses' capacities are dispersed through connections that connect cylinders to their hydraulic system. Under centered load conditions, these connections may absorb up to twice their press' capacity; under off-center loading conditions however, this capacity decreases considerably; therefore, accurately calculating an available tonnage curve is critical in order to protect both connection and tooling components.

Piston Diameter

When running a press, knowing the piston diameter is critical in calculating force output. Measuring at an actual bore is preferable over measuring from fittings; otherwise you could receive inaccurate readings. Furthermore, it's essential that there be a clearance specification from the manufacturer and that you measure at a point within its profile for accurate measurements.

Consider not only peak load requirements but also energy requirements over the entire stroke. Drawing or other forming operations which start several inches off bottom dead center require careful analysis to accurately predict total force available compared to peak load requirements. To do this, integrate the force curve from your press manufacturer over its full stroke, giving a comprehensive picture of energy requirements; otherwise you run the risk of your press stalling or stopping short of capacity.

Piston Radius

Assuming you know some basic geometry, you can calculate the total force a hydraulic cylinder exerts using fluid pressure in psi multiplied by piston cross-sectional area - this calculation allows relatively small cylinders to move extremely heavy loads.

To determine piston diameter, measure it with a ruler or tape and record its value before using the following equation A = Pd2/4 where P represents pi (3.14159), the ratio between circle circumference and diameter.

The larger a piston's cross-sectional area is, the greater is its net output force. This is because Pascal's principle states that force applied at one location to confined liquid is transmitted through its entire length at right angles to that location - this gives hydraulic systems their strength; mechanical gain. As its size increases so does mechanical gain (though this doesn't account for frictional losses). On this page we focus on forward tonnage capacity only.