How to Buy a Hydraulic Press
Hydraulic presses are accepted in many industries because of their wide range of use. They are able to produce significant force with perfect regulation through the use of their hydraulic power transmission. Hydraulic presses creates force through the use of fluid pressure on a piston by means of pumps, valves, intensifiers, and accumulators. Present in the hydraulic method of drive transmission is the ability to provide infinite adjustment of stroke speed, length and pressure while staying within the limits of press capacity. Another thing that can be accomplished is full tonnage can be exerted throughout the complete length of the stroke. Hydraulic presses are also widely recommended for extruding and deep drawing operations where applying full tonnage at controlled stroke speed over a long stroke.
Hydraulic press frames come in C-type, straight-side type with tie rods, or open-rod design. Open-rod machines have a bed, crown, slide, and two additional rods with the slide guided by the rods rather than by conventional gibs. This allows the loading and unloading from all four sides of the press.
- Bed - Flat immovable surface that supports the bolster or dies.
- Frame - The main structure of the press that holds the cylinder and the working surfaces.
- Cylinder assembly - Made of a housing, piston, ram, packing, and seals. Piston diameter and oil pressure determines the force of the press.
- Stroke control rod - Can be set for any distance within the stroke capacity of a cylinder. Adjustable cams which use fixed limit switches to set stroke depth or return length.
- Bolster - A plate mounted on the bed.
- Controls - A dual palm button system is usually used. Here, both buttons must be pressed simultaneously to bring the ram down. Circuits are set to raise the ram up if the operator removes his hands. A distance reversal switch is an adjustable cam, a fixed limit switch sets the depth of the stroke at which the ram reversed. A dwell timer sets the length of the dwell at the bottom of the stroke. A pressure reversal switch sets the pres sure at which the ram reverses.
- Heat exchanger - A water circulation system fixed to the oil reservoir, designed to keep oil at proper temperature.
- Throat clearance-the distance from the vertical centerline of the ram to the frame member at the back of the bed. The distance measures the largest diameter piece that can be positioned with the part centerline under the center of the ram.
- Daylight - The vertical clearance from the top of the bed to the underside of the ram in its peak position upwards. It shows the maximum vertical capacity of the press.
- Work height - The distance from the floor to the bolster.
- Selection - Hydraulic presses vary from mechanical in that fluid pressure is used to actuate the slide instead of a rotated crankshaft.
Each method has its own advantages and disadvantages and are better suited for certain operations than others. Hydraulic presses are usually slower in operation than mechanical presses, for this reason they are normally not recommended for the general run of stamping
operations when production speed is the priority. Users considering acquiring a hydraulic press should note that they are generally less economical to operate than mechanical presses that can efficiently perform a specific identical duty. The reason for this is that the hydraulics have no mechanism comparable to the mechanicalís flywheel for storing energy. In a hydraulic system, oil pressure in the
cylinder falls after each stroke and has to be built up again in a short amount of time. This calls for the use of pumps served by motors, and these pumps take a large amount of electric power to operate. This is why the motor on a hydraulic press typically has several times the capacity as the motor on a mechanical press of similar tonnage. Hydraulic drive systems also place great stress on pipe joints, valves, and seals because its sudden release of pressure with each completed stroke coupled with a contraction of the cylinder and its hydraulic conduits. There are three distinct advantages that hydraulic presses offer.
- The tonnages are adjustable from zero to the maximum with an overload protection put in. Relieves wear and tear on dies, tools, and the press frame itself.
- Constant pressure can be maintained throughout the entire stroke on a hydraulic press and implemented at any pre-determined
- Drawing speeds are adaptable. When the user decides on purchasing a hydraulic press, he must select one with the correct tonnage.
- Inspect the press frame carefully, look for cracks, breaks, or welds.
- Check visible hearing surfaces and ram ways for scores, gouges, or excessive wear.
- Look for parallelism between the bed and the ram with a surface gauge. Lower the slide and zero the indicator at a point on the upper die holder. Move the gauge along the lower die holder surface and the indicator will show any deviation from parallelism.
- Inspect all pump parts and valving for excessive wear. If an inspection gauge is not available, then a straight edge should be placed across the bed, if daylight is visible beneath it, a bowed condition is present.
- Before starting the pump, inspect the rotation of the pump motor and its alignment.
- Inspect the setting of the relief valve at low pressure.
- Prime the pump and eradicate the system of all air at all locations.
- Cycle the machine and inspect all controls after determining the power function and control sequence of the system.
- Inspect the piping to make sure the valve ports are connected properly and check valves and free-flowing in the right direction.
- Inspect for evidence of oil leaks, especially on the ram, around relief valves, at pipe joints and hose connections, and on limit switches and control wiring.
- If accumulators are used, look for broken bladders and low pressure.
- If heat exchangers are used, they should be examined for leaks and ability to hold the temperature below 150 degrees.
- Run the machine, listen carefully for any adverse noise near the piston, pump, and motor bearings.
- After an hour of operation, inspect the filters and strainers for any build-up.
This is one article in a series of How to Buy Metalworking Equipment. Each article showcases and explains a particular type of metalworking machine. They were originally published in the Metalworking Machinery Mailer published by the Tade Publishing Group.
Links to other articles in this series:
How to Buy Automatic Screw Machines |
How to Buy a Press Brake |
Understanding CNC Machining and their Controls |
How to Buy Shears |
How to Buy Saws |
How to Buy a Horizontal Boring Mill |
How to Buy a Hydraulic Press |
How to Buy Shapers |
How to Buy Low-Cost CNC |
Improving your older machines |
How to Buy Straight-Side and Mechanical Presses |
How to Buy Drilling Machines |
How to Buy a Vertical Boring Mill |
How to Buy a Broaching Machine
How to Buy a Hydraulic Press