Skip to content

Hydraulic Press Process Glossary

What are the parts of a press?

If you work in manufacturing, you have likely seen a few hydraulic presses. Fabricators use them in all types of operations, such as bending, forming, and pressing. Recyclers use them to crush cars, and machine shops press bearings and do stamping with them. A hydraulic press can be indispensable in most industrial environments.

The hydraulic press is made up of two cylinders that are connected. While both cylinders contain hydraulic fluid, one cylinder is larger than the other. The larger cylinder is called the ram, and the smaller one is known as the plunger. 

Liquids are hard to compress, making them ideal in a hydraulic press. The hydraulic press works by exerting a small amount of force onto the plunger, pressing the fluid below. The pressure is distributed evenly and, in turn, raises the ram. The pressure between the plunger and the ram works to crush anything placed between them. 

While those who work with hydraulic presses may understand them thoroughly, those outside the industry might find some of the terminology confusing. Although it might not be necessary to comprehend every term associated with hydraulic presses (and there are a lot of them!), here are some of the standard hydraulic press terminologies that everyone should know:

36 very professional hydraulic press process glossary

  1. Bed: The press bed is the flat, stationary, machined surface that supports the lower bolster or dies.
  2. Bed Cushion: Commonly required for deep draw applications, a bed cushion is used to apply resistance force when pushed upon. This resistance force ensures the material is drawn smoothly, without wrinkling or tearing. Bed cushion force can be dynamically controlled throughout the stroke, allowing the resistance force to change based on the position of the bed cushion.
  3. Bed Height: The bed height on a press machine is the distance from the bottom of the press structure to the working height or the top of the bed bolster. If a press requires a pit, the working bed height could be defined as the distance from the floor to the top of the bed bolster.
  4. Bolster: The bolster is the removable plate that serves as the working surface of a press. The plate is typically bolted to the bed and ram weldments. Tooling is attached to the bolster, which can feature a variety of work-holding features such as T-slots, drilled and tapped holes, lift rails to accommodate quick die change systems and more.
  5. Bushing: Found on post-guided presses, the bushing is a fixed or removable cylindrical metal lining used to guide the ram and reduce friction. Graphite-impregnated bronze bushings that do not require external lubrication are the longest-lasting type of bushing.
  6. Crown: The crown on a press is the upper structural weldment containing cylinders that drive the motion of the ram.
  7. Cycle: A typical press cycle is the complete movement of the ram, from the initial start position back to the same start position, which may include feeding and removal of the material or workpiece(s).
  8. Frame: The main structure of the press contains the cylinder(s) and the working surfaces.
  9. Cylinder: The cylinder is the main actuator of a press. This mechanical actuator converts pressure into linear movement, creating force.
  10. Daylight: Also commonly known as the open height of a press, daylight is the distance between the bed bolster and the ram bolster when the ram is fully retracted.
  11. Deflection: Deflection is the amount of deviation from a straight line that occurs when force is applied to the structure of a press. In c-frame press designs, this is often referred to as yawning and expresses the amount a frame flexes under a load.
  12. Dwell: Dwell is the amount of time required for a press to maintain pressure during a cycle. This is typically accomplished by using pressure lock valves or variable volume pumps that are remotely controlled during long periods of precise pressure holding. Some presses use pneumatic dwell systems, powered by a pneumatic pump integrated into the hydraulic circuit, to reduce noise output and energy consumption.
  13. Finite Element Analysis (FEA): Finite Element Analysis is a computerized method for predicting how a press’s structure will react to real-world forces such as vibration, heat, fluid flow, etc. Performed during the press engineering phase, FEA works by breaking down a real object into finite elements and using mathematical equations to predict the behavior of each element.
  14. Gibs: Gibs are adjustable mechanisms that guide the reciprocating motion of the ram to ensure squareness and parallelism throughout the cycle. Gibs are usually provided with replaceable liners and are adjustable front-to-back as well as left-to-right to enable the setting of proper clearance and to compensate for wear.
  15. Heated Platens: Heated platens are steel plates to which the press’s tooling is attached that are heated using water (steam), oil, or electricity. These systems usually require thermal breaks (insulation) between the platens and the press structure. Heating controls can be separate or fully integrated into the press control system.
  16. Human-Machine Interface (HMI): A Human Machine Interface (HMI) is a programmable touchscreen computer that initiates the press cycle, monitors press health, and logs critical data and cycle parameters.
  17. Hydraulic Power Unit (HPU): The Hydraulic Power Unit (HPU) on a press is a system comprising the tank, motor, hoses, pumps, and chillers that work in unison to create pressure. It is the mechanism that applies pressure to drive motors, cylinders, and other parts of the system. HPUs are typically either skid-mounted or crown-mounted on the press, depending on the footprint requirements.
  18. Motor: The motor on a press is the electric machine that transforms fluid energy into rotary energy.
  19. Platen: Platens are steel plates, sometimes heated, that are attached to a moving or stationary press member.
  20. PSI: PSI is an abbreviation for Pounds per Square Inch, a unit for measuring pressure in a hydraulic press.
  21. Pump: A pump is a device that converts mechanical force and motion into hydraulic fluid power.
  22. Ram: The ram (or slide) is the middle weldment on a press that slides within the frame to create pressure on the tool or die. The ram can move vertically or horizontally depending on the press configuration. Some multi-action presses even have multiple rams for complex forming processes.
  23. Ram Knockout: A ram knockout is an ejection device required by many press operations that strips the formed part from the punch or die.
  24. Ram Speed: Ram speed on a hydraulic system is the total time it takes for the ram to move from the open to the closed position, measured in IPM (inches per minute). Speed is also commonly measured at the three distinct stages of the stroke:
    1. Fast Approach Speed lowers the ram quickly during the portion of the stroke that does not require any force.
    2. Pressing Speed, commonly referred to as the “working portion” of the stroke, is when force is required and speed is slower.
    3. Stripping/Retract Speed is after the Pressing portion of the stroke is completed when the ram retracts at a Fast Retract speed which affords little force.
  25. Return on Pressure: Return on pressure is a programmable cycle parameter that uses an adjustable pressure sensing device (transducer) to determine the desired maximum pressure to be achieved by the ram. Once this pressure is achieved, the ram completes the cycle by returning to the “Home” (or “Up Limit”) position.
  26. Return on Position: Return on position is a programmable cycle parameter on a press that uses a position sensing device (transducer) to determine the desired position to be achieved by the ram. Once this position is achieved, the ram completes the cycle by returning to the “Home” (or “Up Limit”) position.
  27. Shut Height: Shut height is the distance between the bed bolster and the ram bolster when the ram is fully extended. This is commonly known as the “Closed Height.”
  28. Stroke: Stroke is the total distance the ram can travel, from full extension to full retraction.
  29. Stroke Control: Most presses feature Adjustable Retract Limit Switches to restrict the retract distance of the ram (also known as the “Up Limit” Position). Using only the required stroke for part loading and unloading can shorten cycle times. Other programmable limits may include Slow Down Limit for deceleration from Fast Speed to Slow Speed; Bottom Stop Position and/or Bottom Stop Pressure.
  30. T-Slot: A t-slot is a notch machined into the platens of a press to hold the die in place. This work-holding feature also facilitates quick die changes.
  31. Throat Clearance: On c-frame (gap-frame) presses, the throat clearance is the distance from the vertical centerline of the bed to the back of the press behind the bed. This measure is required to determine the diameter of parts and tools that can be positioned within the press.
  32. Tie Rod: Tie rods on a 2-post or 4-post press are long rods with threads and nuts on both ends that hold the frame members together. These rods are stretched to place the frame members under compressive load.
  33. Tonnage: Tonnage is the maximum amount of force a press machine can exert, typically called out in U.S. tons.
  34. Transducer: A transducer is a device that measures the linear position or pressure of the ram or cylinder rod.
  35. Weldment: Weldments are structural components formed by welding together steel plates. Most modular presses have three main weldments: crown, bed, and ram.
  36. 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.

Other hydraulic press terms

  1. Blankholder – A control controlled force to hold the edges of the blank in deep drawing operations. Similar to a die cushion.
  2. 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.
  3. Distance Reversal Switch – An adjustable limit switch to set the depth of stroke at which the ram reverses.
  4. 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 movements.
  5. Heat Exchanger – A device attached to the oil reservoir to circulate water or air to keep oil at proper operating temperature.
  6. Knockout – A device to strip the part from the punch or die.
  7. Platen – A plate-sometimes heated-attached to a moving or stationary press member.
  8. Pressure Reversal Switch – An adjustable switch to set the pressure at which the ram reverses.

Cited Articles: Press anatomy and terminology. What are the parts of a press?