Print This User's Guide
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.
PRINCIPAL PARTS
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 mechanical’s
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 position.
Drawing speeds are adaptable. When the user decides on purchasing a hydraulic
press, he must select one with the correct tonnage.
INSPECTION
NON-POWER
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.
UNDER POWER
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.
