PRESSES, STRAIGHT SIDE
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Details to look for during new and used machinery inspection:
Tonnage capacity
Stroke, inches
Throat depth, inches
Daylight, ram up, inches
Length & width of table
Ram dimensions
Speed of ram, down, I.P.M
Speed of ram, up, I.P.M.
Type of press - up moving/down moving
Pump & tank information/size, capacity, etc.
ACCESSORIES
Bolster plate, inching or rapid traverse, die cushions
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.
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.
