PRESSES, STRAIGHT SIDE
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Details to look for during new and used machinery inspection:
Tonnage capacity
Strokes per minute
Diameter of crankshaft at bearings/ crank pins
Stroke of slide
Adj. of slide
Area of slide face
Area of bed
Opening in bed
Throat gap
Distance between uprights
Window in uprights
Shut height
Open height
ACCESSORIES
Cushions, air counterbalance, bolster plate, gear
guards, inching device roll feed, scrap cutter, safety
guards, surge tank, reinforcing tierods, subtable
How to Buy Straight-Side and Mechanical Presses
Presses are used in the fabrication of metal. Mechanical presses operate through the use of a flywheel. A flywheel stores energy which is then released and transferred to the main slide by the use of mechanisms such as a crank, eccentric, knuckle joint or toggle. The stroke of the slide is adjustable within the limits of daylight in a mechanical press. They are also classified by the number of slides or ram they have (either single, double or triple action). Double-action has two slides moving in the same direction against a fixed bed. Triple-action has three moving slides, two moving in the same direction with a third moving upward through the fixed bed in an opposite direction. The slide movement in a single-action press is similar to that of the inner slide of a doubleaction press. Straight-side presses are classified into 3 types: single-point, two point and four point, referring to the number of connections between the slide and the main shaft. They also have their drive mechanism located above the bed unless there is an underdrive mechanism present, which drives below the bed and pulls the ram downward.
PRINCIPAL PARTS
The principal parts of a single-action straight-side
press are as follows:
Bed - the foundation of the press, providing
rigid stability and support for all other
parts and the dies.
Uprights - columns that support the crown
of the press. They direct the slide in precision
gibs as it compensates and houses the press-controls
and counterbalance cylinders.
Crown - houses the press-drive mechanism
and joins the uprights at the top of the
machine for rigidity.
Tie Rods - made of heavy tension to tie
the crown and the bed together, resisting
forces that tend to separate them as the
slide closes. They are located at each corner
of the press and pass completely from top
to bottom.
Crankshaft - in a fixed position rotates
and through gearing causes the pitman and
the slide to reciprocate.
Pitmans - used in connecting the crankshaft
and the slide.
Flywheel - rotates to provide power to the
slide. Generally quill-mounted to aid shafts
and bearings of their weight and to minimize
wear.
Clutch - the coupling used to connect and
disconnect the flywheel from the main shaft.
Brake - stops the press after clutch is
disengaged. Brakes are generally continuous,
except on larger presses they are releasable.
CLASSIFICATION OF PRESSES
1. Source of power.
2. Method of actuating
the slide.
3. Type of frame.
4. Type of
brake & clutch.
These are the four types
of characteristics of a mechanical press.
Source of power - Mechanical presses use
a flywheel for energy which is transferred
to the main slide through the use of gears,
crankshafts, eccentrics or levers. The energy
stored inside the flywheel must be adequate
enough to ensure the work per stroke required
of the press and will not reduce the flywheel’s
speed by more than 10% to 15%.
Method of Actuating the Slides - Mechanical
presses usually utilize a crankshaft for
ram movement, its bearings ride in bushings
supported by the press frame. Some mechanical
presses also use eccentrics, instead of
crankshafts for obtaining ram movement.
The eccentrics are usually mounted directly
on the main drive gear. Eccentric drives
offer more surface area of bearing support
for the connecting rod, minimizing the strain
of backthrust when the die is closed. However,
in order to increase the press stroke on
eccentrics, the offset must be increased
in diameter, making it almost impractical.
Type of Frame - A straight-side press utilizes
a slide which travels downward between two
straight sides, these presses are usually
used for heavy work.
Type of Brake & Clutch - Presses require
a brake to stop the press when the clutch
is disengaged. Small presses with positive
clutches means the brake is usually on continuously.
On larger presses the brake is releasable.
Clutches are either positive, friction,
or eddy-current type. Apositive clutch establishes
a direct connection between the crankshaft
and drive through the use of a hardened
metal part which connects a mating part
on the flywheel or gear. A friction clutch
transmits flywheel motion to the shaft by
friction instead of direct mechanical engagement.
Most friction clutches utilize air pressure,
but mechanical means are sometimes used.
An eddycurrent type clutch relies upon electrical
means instead of friction to transmit motion.
OTHER TYPES OF STRAIGHT-SIDE PRESSES
There are other types of straight-side presses
that are more specialized to perform various
stamping operations. Dieing machines are
inverted presses where the drive mechanism
is under the bed.
The press is able to cycle its ram at faster
speeds since the ram mass on the machine
is small. Also, the underdrive mechanism
maximizes die access and allows stability
and rigidity. Transfer Presses are straight-side
tie-rod presses large enough in die area
and tonnage capacity to allow mounting and
are able to transfer work automatically
from die to die between strokes. Therefore,
these presses can replace a number of separate
single-operation presses including all the
material handling equipment needed for these
sequenced operations. Quick Die-Changing
Presses are different from conventional
presses in that they are equipped with interchangeable
bolster plates set up to move laterally
in and out of the press on tracks. Dies
are changed here without any interruption
in production.
SELECTION
When selecting a mechanical press you should
look at tonnage and energy capacity, stroke
length, shut height and strokes per minute
considering your individual production requirements.
Apress rated tonnage is the maximum force
that should be exerted by the slide against
the work at a given distance above the bottom
of the stroke. The higher the rating is
the greater the torque capacity of its drive
members and its capability of delivering
more flywheel energy. Presses with flywheel-type
drives are basically used for light blanking
and piercing operations. The energy requirements
of these machines are small and operate
at relatively high speeds. Single-geared
presses are mostly used for shallow draw-work,
requiring more energy than flywheel types.
Double -geared presses are used for deeper
draw operations when a larger amount of
energy is needed.
INSPECTION
NON-POWER
Check press frame for cracks, breaks, or
welded repairs.
Check all bearing surfaces, look for scores,
gouges, or excessive wear.
Check the gibbing, make sure adequate adjustment
is there. It’s important in maintaining
slide alignment.
Look for parallelism with a surface guage
between the bed and the ram, deviation should
not exceed 0.001 in/ft.
Check the bearings with a jack and indicator,
determine if excessive wear is present on
the bottom of the ram.
Visually check the gears for broken teeth
or excessive wear.
UNDER POWER
Turn on the machine and check all controls.
Listen to the gears, make sure there are
no unusual or grinding sounds.
Make sure there are no unusual sounds in
the clutch, listen for slippage.
If air operated, make sure the clutch is
supplied with the adequate amount of pressure.
Check joints closely on presses with tie-rod
construction, check for excessive wear.
Finally, make sure the ram comes down precisely
and smoothly, checking for any improprieties.
*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.
