Equipment MLS: How to Buy Straight-Side and Mechanical Presses

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.

Links to other articles in this series: