How to compute tonnage requirements:
1. General - When pressure per square inch is known:
psi x area of work/2000 = 2 tons of ram force required
Example: Where it is known that 100 psi is needed to do a job on a 5" x 8" wide piece.
100 x 5" x 8"/2000 = 2 tons
2. Press Fit - To determine the force required to press fit two round pieces together such as a
shaft pressed into a bushing, use the following formula:
F = D x ϖ x L x I x P/2
Where:
F = force required in tons
D = diameter of the part to be pressed in inches
L = length of part to be pressed in inches (Note: the length of the interference fit only.)
I = interference in inches (usually .002" to .006")
P = pressure factor (See table below).
Diameter Pressure Diameter Pressure Diameter Pressure Diameter Pressure
(inches) Factor (inches) Factor (inches) Factor (inches) Factor
1 500 3 156 5 91 7 64
1¼ 395 3¼ 143 5¼ 86 7¼ 61
1½ 325 3½ 132 5½ 82 7½ 59
1¾ 276 3¾ 123 5¾ 78 7¾ 57
2 240 4 115 6 75 8 55
2¼ 212 4¼ 108 6¼ 72
2½ 189 4½ 101 6½ 69
2¾ 171 4¾ 96 6¾ 66
Example: A steel shaft 2" in diameter pressed into a hole 3" long. The interference fit between
the two diameters is .006".
2" x 3.14 x 3" x .006" x (240/2) = 13.56 tons
3. Punching - A quick guide to determine tonnage requirements for punching steel is:
Diameter x thickness x 80 = tons (where 80 is constant for steel. Use 65 for brass.)
Example: A 3" hole in .250" stock: 3" x .250" x 80 = 60 tons
For noncircular holes, instead of the diameter, use 1/3 of the total length of cut.
Example: A rectangular hole 4" x 6" in .250" stock: (4" + 6" + 4" + 6"/3) x .250" x 80 = 133.3
tons
4. Deep Drawing - Deep-drawing calculations can be complex. The press, dies, material, radius,
and part shape all have bearing. For drawing round shells, the following formula is a simple
guide:
C x T x Ts = tons
Where:
C = circumference of the finished part; T = material thickness in inches; and
Ts = tensile strength of the material.
Example: To draw a 5" diameter cup of .040" stock with a tensile strength of 46,000 psi would
require the following tonnage:
(5 x 3.1416) x .040 x (46000/2000) = 14.44 tons
A 20-ton press would be recommended
5. Straightening - The pressure required to straighten a piece of metal depends on its shape.
Below is an approximate formula with a further definition for different shapes.
Where F is the ram force in tons; 6 is a constant; U is ultimate strength of the material in psi; Z
is the section modulus (see below); and L is the distance between the straightening blocks in
inches.
Example: A 2" diameter shaft, 18" between the blocks, 100,000 psi ultimate strength.
How to determine strokes per minute for a hydraulic press
The number of strokes per minute for a hydraulic press is determined by calculating a separate
time for each phase of the ram stroke. The rapid advance time is calculated, then the pressing
time, (the work stroke); then, if there is no dwell time, the rapid return.
The basic formula for determining the length of time in seconds for each phase of the stroke:
Example: a hydraulic press with a 600 IPM rapid advance, 60 IPM pressing speed, and 600 IPM
rapid return. The work requires a 3" advance, 1" work stroke, and 4" rapid return.
60 ÷ 2.199 = 27 cycles per minute.
* Electrical actuation and valve shift time varies depending on the type of hydraulic circuit. One half
second is a reasonable average figure.
1. These formulae are intended as guidelines only. Please consult a qualified manufacturing engineer for
recommendations concerning your specific requirements.
2. Based on steel shaft and cast iron bushing (with OD/ID > 2).
1. Tonnage. Is the tonnage required to do a job the same for a hydraulic press as it is for a
mechanical press? The answer is yes. There is no real difference. The same formulae are used
to determine tonnage. The tooling is usually interchangeable. There may be certain applications
such as deep drawing where the full power stroke characteristic of a hydraulic press reduces the
tonnage, but there are no known instances where using a hydraulic press requires more
tonnage.
Selecting press tonnage in the typical press room is often little more than guesswork. If, for
example, a job is successful on a 100-ton mechanical press, it tends to stay there for the life of
that job. The job may never have been tried at 75 tons or at 50 tons.
With a hydraulic press, however, you can adjust tonnage quickly and easily, tuning the press to
precisely the right tonnage for each specific job.
2. The action of the machine. Even though the tonnage question might be settled, the question
of the effect of the stroke on the work is often asked. Is it the same as with a mechanical press?
The answer, again, is yes in most cases. There are some specific limitations. Drop hammers and
some mechanical presses seem to do a better job on soft jewelry pieces and impact jobs. The
coining action seems sharper if the impact is there.
In deep drawing, however, the full power stroke of a hydraulic press produces significantly
better results.
Otherwise there are very few examples where the application of 100 tons of hydraulic force
produces any significant difference in the character of the part given the same tooling.
Shear in the dies will reduce blanking tonnage for hydraulic presses in the same way it does for
mechanical presses.
3. Type of press selection. Open-gap presses provide easy access from three sides. 4-column
presses insure even pressure distribution. Straight-side presses offer the rigidity required for
off-center loading in progressive die applications.
The more critical the work and the more demanding the tolerances, the greater the reserve
tonnage capacity should be.
4. Accessories. Most hydraulic press builders offer a wide array of accessories. These commonly
include:
Distance reversal limit switches
Pressure reversal hydraulic switches
Automatic (continuous) cycling
Dwell timers
Sliding bolsters and rotary index tables
Die cushions
Ejection cylinders or knockouts
Electronic light curtains and other devices
Touch screen controls
Servo system feedback for precise, consistent, repeatable stroke control
5. Quality. The industry offers various levels of quality. There are light-duty presses that are
capable of "spanking" the work momentarily and reversing, and there are heavy-duty machines
designed for general purpose metalworking applications.
Here are just a few construction points that will provide a basis for comparison of one machine
with another:
1. Frame. Look at frame construction-rigidity, bolster thickness, dimensional capacity,
and other factors.
2. Cylinder. What diameter is it? How is it constructed? Who makes it? How serviceable is
it?
3. Maximum system pressure. At what psi does the press develop full tonnage? The
most common range for industrial presses is 1000 to 3000 psi.
4. Horsepower. The duration, length, and speed of the pressing stroke determines the
horsepower required. Compare horsepower ratings.
5. Speed. See page 5 to determine the speed of a hydraulic press.
6. Speed. There are no hydraulic presses today that are as fast as the fastest mechanical presses.
If speed is the sole requirement and the material feed stroke is relatively short, the mechanical
press remains the best selection.
7. Stroke depth. If a limit switch is used to determine the bottom, the stroke depth is not likely to
be controlled much closer than .020".
Many hydraulic presses can be set to reverse at a preselected pressure, which usually results in
uniform parts.
Generally, if absolute stroke depth accuracy is required, "kiss" blocks must be provided in the
tooling.
However, Greenerd hydraulic presses are now available with an accurate built-in method of
limiting the down stroke. Greenerd's new closed-loop servo-hydraulic system dramatically
improves stroke depth control, guaranteeing consistent, repeatable results. In many
applications, this system eliminates the need for "kiss" blocks.
8. Automatic feeding equipment. Hydraulic presses require some external or auxiliary power to
feed stock. The feeder must have its own power, and must be integrated with the press control
system.
There is, however, an increasing selection of self-powered feeding systems available: roll feeds,
hitch feeds, and air feeds.
9. Shock after breakthrough in blanking. Both mechanical and hydraulic presses experience
this problem. But, the hydraulic system of a hydraulic press must also be isolated from the
shock associated with decompression. If the hydraulic system does not contain an antishock
feature, this shock can affect the lines and fittings.
