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Tonnage and Energy

By: Peter Ulintz

Monday, September 01, 2008
 
It is essential to understand that press tonnage and press energy are not one in the same. The tonnage rating of any press is the largest load, in tons, that the press can withstand without causing structural damage to the machine frame, slide-adjusting mechanisms, pitman (connection rods) or main gear bushings. The energy rating of a press is a function of applied press loads and the distance through which the loads are applied.

Unlike hydraulic presses, mechanical presses have their full rated force available only near the very bottom of the press stroke. Cutting, bending, embossing and other operations that generally occur near the bottom of the stroke can take advantage of the full rated tonnage capacity of a mechanical press.

Deep-drawing operations, on the other hand, tend to start several inches above the bottom of the press stroke. Since the working capacity of a mechanical press is a function of its structural design, gearing, motor size, flywheel mass and other variables, the machine’s maximum working capacity (tonnage) is a fixed value. When working distances increase, as with deep-drawing operations, the

Fig. 1 Press-Force Curve
Fig. 1
available force higher up in the press stroke will be less. At the risk of over-simplification, this can be expressed as W = f * d, where W is the maximum working capacity of a press in tons, f is the force applied during deformation and d is the distance through which the force is applied.

For instance, deep drawing 3 in. off bottom in a 600-ton mechanical press provides less than 50 percent of the rated machine capacity to carry out the drawing operation. De-rated tonnage is a term used to describe this reduction in available force.

The press crankshaft in a mechanical press rotates, which causes the mechanical advantage of the pitman to change constantly. This means that the relationship between force and distance will not be linear. As a result, mechanical-press manufacturers supply force curves that depict available force versus the distance of the slide from the bottom of the stroke. A simplified press-force curve is shown in Fig. 1.

The energy rating of a press also is a function of applied press load and the distance through which the load is applied. For example, pushing 200 tons through 3 in. of deep drawing requires 600 in.-tons of energy. Changing the part material to high strength steel could require 500 tons of force working through the same 3-in. distance—expending 1500 in.-tons of energy.

Fig. 2 Press-Energy Curve
Fig. 2

Depending on the type of presswork performed, each stroke of the press will expend a given amount of energy, all of which needs to be replaced. This requires critical attention and focus on the size of the main drive motor (horsepower) and the rotational speed of the flywheel.

The main motor, along with its electrical connections, is the only source of energy for the press and it must have sufficient horsepower to supply the demands of the stamping operation. The press flywheel is an energy-storage device. The flywheel must be able to store and deliver the required energy when needed, without excessive slowdown. The amount of energy stored in the flywheel depends on its mass and rotational speed. The stored energy varies by the square of the speed; thus, a large amount of energy can be stored in the flywheel when the press is running at full speed. The graph in Fig. 2 shows an energy-capacity curve for a 600-ton press. This graph illustrates how the available energy of this press diminishes to 25 percent of rated capacity when its speed is reduced from 24 strokes/min. to 12 strokes/min. At these slower speeds, flywheel size becomes the limiting factor for delivering the energy requirements to the press.

As should be apparent now, it is entirely possible to have a mechanical press well within the tonnage requirements for your particular stamping but woefully short on the energy needed to carry out production. MF

 


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