For over 30 years, there has been a clear trend toward hydraulic presses. Until recently, the most common production press has been the mechanical press-a force-delivering machine in which power is derived from a rotated crankshaft. In 1991, shipments of hydraulic presses surpassed mechanical presses. The hydraulic press has become a major factor in the U.S. press market and is even more popular in Europe. U.S. Shipments of Hydraulic and Mechanical Presses (1990-1995) Source: U.S. Department of Commerce

Faster and More Reliable than Ever

Today's hydraulic presses are faster and more reliable then ever. In the last decade, the technology has gone through constant change. Improvements in seals, more efficient pumps, and stronger hoses and couplings have virtually eliminated leaks and minimized maintenance.

Programmable logic controllers (PLCs) and other electronically-based controls have improved speed and flexibility. With new computer interfaces and monitoring, hydraulic presses are now widely used in advanced computer-integrated manufacturing systems.

More Productive on Hand-Fed Jobs

Mechanical presses are often faster in automatically fed, short stroke, and short feed-length blanking operations.

But, despite the automation trend, the vast majority of jobs are still hand-fed. The lot size of many jobs is simply too small to justify the expense of automatic feeding. With markets changing so rapidly, many manufacturers are reluctant to make large investment in automatic feeding equipment. Just-In-Time (JIT) manufacturing has further increased the incidence of short runs.

So, hand feeding remains the primary feeding method, and hydraulic presses offer obvious competitive advantages on hand-fed jobs.

Faster Setups and Changeovers

Users of hydraulic presses are often amazed at how quick they can change dies and get on with a new job. It's especially true if they're used to the highly critical job of setting the stroke for a mechanical press. Hydraulic presses eliminate this very tricky, time-consuming task. Because hydraulic presses maintain full tonnage throughout the entire length of their stroke, there is no need to determine the exact location of maximum tonnage.

On hand-fed jobs, the floor-to-floor or bench-to-bench time is often the same for mechanical and hydraulic presses. So, faster setups and greater up time make hydraulic presses more productive than mechanical presses.

Flexibility for a Wide Range of Applications

Lines of hydraulic presses are showing up in increasing numbers on high volume jobs. The jobs listed below, and hundreds of others, are being done on hydraulic presses today.

• Electric motor manufacturers assemble motor shafts to rotors, compress laminations, and press cores into housing.
• Automotive manufacturers press tiny shafts into water pumps, assemble shock absorbers, blank and form diaphragms, and stake disc brakes together.
• Jewelers coin Boy Scout pins.
• Frozen fish blocks are shaped for more efficient processing.
• Aircraft companies form tough titanium housings.
• Tuba bells and cymbals are shaped in huge forming presses.
• Hardened road grader blades and machine ways are straightened.
• Hollowware manufacturers blank and draw brass bowls automatically from coiled stock.
• Computer disc shafts are pressed into precision bearings.

Full power stroke - The full power of a hydraulic press can be delivered at any point in the stroke. Not only at the very bottom, as is the case with mechanical presses. Advantages? No allowances for reduced tonnage at the top of the stroke. In drawing operations, for example, you have the full power of the press available at the top of the stroke. You don't have to buy a 200-ton press to get 100 tons throughout the stroke. Other advantages are faster set-ups and no time consuming job of adjusting the stroke nut on the slide to accommodate different dies.

Built-in overload protection - A 100-ton hydraulic press will exert only 100 tons pressure (or less, if you have set it for less) no matter what mistakes you make in set-up. You needn't worry about overloading or breaking the press or smashing a die. When hydraulic press reaches its set pressure, that's all the pressure there is. The relief valve opens at that limit and there is no danger of overload.

Much lower original cost and operating costs - Hydraulic presses are relatively simple, and you may be surprised at the significant cost advantage over mechanical presses in comparable sizes. The numbers of moving parts are few, and these are fully lubricated in a flow of pressurized oil. Breakdowns, when they occur, are usually minor; not, for example, like a broken crankshaft. Replacements of packing, solenoid coils, and occasionally a valve, are typical maintenance items. Not only are these parts inexpensive, but also they are easily replaced without tearing the machine apart. This means more up-time and lower maintenance costs.

Larger capacities at lower cost - It is easier and less expensive to buy certain kinds of capacity in hydraulic presses. Stroke lengths of 12, 18, and 24 inches are common. Extra stroke length is easy to provide. Open gap (daylight), too, can be added without much additional cost. Similarly, larger table areas and small presses with big bed areas can be provided. Large 200-ton presses with relatively small beds are available; tonnage of the press doesn't dictate what the bed size will be.

More control flexibility - Hydraulic press power is always under control. The ram force, the direction, the speed, the release of force, the duration of pressure dwell, all can be adjusted to fit a particular job. Jobs with light dies can be done with the pressure turned down. The ram can be made to approach the work rapidly, then shifted to a slower speed before contacting the work. Tool life is thus prolonged. Timers, feeders, heaters, coolers, and a variety of auxiliary functions can be brought into the sequence to suit the job. Hydraulic presses can do far more than just go up and down, up and down.

Greater versatility - A single hydraulic press can do a wide variety of jobs within its tonnage range. Commonly seen are deep draws, shell reductions, urethane bulging, forming, blank and pierce, stake, punch, press fits, straightening, and assembly. They are also used for powered metal forming, abrasive wheel forming, bonding, broaching, ball sizing, plastic and rubber compression, and transfer molding.

Quiet - Fewer moving parts and the elimination of a flywheel reduce the overall noise level of hydraulic presses compared to mechanical presses. Properly sized and properly mounted pumping units meet and exceed current Federal standards for noise, even with the pump under full pressure.

Because each phase of the ram movement can be controlled, noise levels can also be controlled. A hydraulic ram can be controlled to pass through the work slowly and quietly.

More compact - A typical 20-ton hydraulic press is eight feet high, six feet deep, and two feet wide. A 200-ton press is only ten feet high, nine feet deep, and a little over three feet wide. At ten times the capacity, the 200-ton press only takes up 50 percent more floor space. Hydraulic presses become less and less expensive compared to mechanical presses.

Lower tool costs - the built-in overload protection (see advantage 2) goes for the tools, too. If they are built to withstand a certain load, there is no danger of damaging them because of overloading. Tools can be sized to withstand the load of a particular job, not a particular press. The pressure of the press can be set down to suit the job. The lack of impact, shock, and vibration promotes longer tool life.

Safety - No manufacturer will (or should) claim that hydraulic presses are safer than mechanical presses. Both types of machines are designed and built to be safe if the controls and safety features built in are used properly.

Improperly used, all machines are potentially dangerous. But the factor of control of the ram movements makes hydraulic presses easy to make safe. Non-tie down, anti-repeat, dual palm button controls are used. The interlocking of guards, as well as other safety devices, is relatively easy because of the nature of a hydraulic press control system.

1. Cylinder - Cylinder assembly consists of a cylinder, piston, ram, packing, and seals. Piston diameter and oil pressure determine the force (tonnage) that a given press can deliver. 2. Frame - The main structure of the press containing the cylinder(s) and the working surfaces. 3. Stroke Control - Stroke length can be set for any distance within the stroke limits of the cylinder. Adjustments include: top of stroke, pre-slowdown point, and bottom of stroke. 4. Throat Clearance - The distance from the vertical centerline of the ram to the frame member behind the bed. This distance determines the largest diameter piece that can be positioned with the part centerline under the center of the ram. 5. Daylight - The vertical clearance from the top of the bolster to the underside of the ram in its maximum up position. This term sometimes is confused with the mechanical press term "shut height". Shut height is the clearance over the bed with the ram full down. "Daylight" describes the maximum vertical capacity of the press. 6. Bolster - A plate or structure mounted on the bed. Hydraulic press manufacturers provide a removable bolster on most models. 7. Bed - Flat, stationary machined surface that supports the bolster or dies. 8. Dual Palm Button Controls - A common method of actuating hydraulic presses. Both buttons must be depressed at the same time to bring the ram down requiring the operator to use both hands. Control circuits include non-repeat and anti-tie down features. 9. Work Height - The distance from the floor to the top of the bolster.

Other hydraulic press terms Blankholder - A control controlled force to hold the edges of the blank in deep drawing operations. Similar to a die cushion. Die Cushion - A hydraulic or air cylinder positioned below the bolster and bed, providing uniform blank holding in deep drawing. Cushions also strip finished parts from the punch or die. Distance Reversal Switch - An adjustable limit switch to set depth of stroke at which ram reverses. Dwell Timer - An adjustable timer to set the length of dwell at the bottom of the stroke. The timer may be used for other functions such as timing a sequence of press movement. Heat Exchanger - A device attached to the oil reservoir to circulate water or air to keep oil at proper operating temperature. Knockout - A device to strip the part from the punch or die. Platen - A plate-sometimes heated-attached to a moving or stationary press member. Pressure Reversal Switch - An adjustable switch to set the pressure at which ram reverses.

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).
Pressure Factors²

Diameter (inches)	Pressure Factor	Diameter (inches)	Pressure Factor	Diameter (inches)	Pressure Factor	Diameter (inches)	Pressure 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

3.
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

4. 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

5. 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.

6. 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

7. 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:

o Distance reversal limit switches

o Pressure reversal hydraulic switches

o Automatic (continuous) cycling

o Dwell timers

o Sliding bolsters and rotary index tables

o Die cushions

o Ejection cylinders or knockouts

o Electronic light curtains and other devices

o Touch screen controls

o 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:

a. Frame. Look at frame construction-rigidity, bolster thickness, dimensional capacity, and other factors.

b. Cylinder. What diameter is it? How is it constructed? Who makes it? How serviceable is it?

c. Maximum system pressure. At what psi does the press develop full tonnage? The most common range for industrial presses is 1000 to 3000 psi.

d. Horsepower. The duration, length, and speed of the pressing stroke determines the horsepower required. Compare horsepower ratings.

e. Speed. See page 5 to determine the speed of a hydraulic press.

1. 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.

2. 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, the hydraulic presses are now available with an accurate built-in method of limiting the down stroke. 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.

3. 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.

4. 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.