Mechanical Presses

In most mechanical P/M compacting presses, electric motor-driven flywheels supply the main source of energy used for compacting and ejecting the part. The flywheel normally is mounted on a high-speed shaft and rotates continuously. A clutch and a brake mounted on the flywheel shaft initiate and stop the press stroke. To initiate a press stroke, the brake is disengaged and the clutch is engaged, causing the energy stored in the rotating flywheel to transmit torque through the press gearing to the final drive or press ram.

Clutch and brake systems should be of the partial revolution type that can be engaged and disengaged at any point in the pressing cycle. The clutch usually is pneumatically engaged with a spring release, and the brake is pneumatically released with a spring set, thereby providing full stopping ability in the event of loss of air pressure. An adjustable speed device normally is supplied with electric drive motor, providing production rate adjustment as indicated by pressing and ejection conditions.

On presses that have main motor capacities up to —19 kW (25 hp), the adjustable speed drive is usually of the variable-pitch pulley or traction-drive type. Above 19 kW (25 hp), direct-current or eddy-current control devices are preferred. The motor and drive must be totally enclosed to prevent contamination by metal powder dust.

Gearing systems usually are either single-reduction (Fig. 1) or double-reduction (Fig. 2) arrangements. Single-reduction gearing frequently is used in lower tonnage presses ( S445 kN, or 50 tons) that have stroking rates of —50 strokes/min. Higher tonnage presses use double-reduction gearing and commonly have maximum stroking rates of 30 strokes/min.

Fig. 1 Single-reduction gearing systems for P/M compacting press
Fig. 2 Double-reduction gearing systems for P/M compacting press

The low-speed shaft of the press, normally called the main shaft, is linked to the press ram, causing motion of the tooling for the compacting and ejection cycles. Ram driving mechanisms can be either cam- or eccentric-driven arrangements.

Cam-driven presses generally are limited to pressing capacities S 890 kN (100 tons). The main shaft of the press has two cams--one cam operates the upper ram, and the other cam operates the lower ram for compacting the part. The cam that operates the lower ram also controls the powder feed into the die and ejects the part from the die after compacting. Cams normally operate linkages that convert the main shaft rotary motion into the linear motion of the tooling.

Figure 3 shows a schematic of a cam-driven press. The cams in this type of press can be adjusted or arranged with removable sections, thus allowing cam motion to be varied to produce special motions to compact the part. Pressure can be applied either simultaneously or sequentially to the top and bottom of the compact. Anvil and rotary presses are types of cam-driven machines. These presses are described in more detail later in this article.

Main ram

Lever arm

'Jpper bearing

Cam ins

Lever arm

Cam ins

'Jpper bearing

Adjusting nuts

Lower bearing

Adjusting nuts

Cam body

Press frame

Fig. 3 Schematic of cam-driven compacting press

Eccentric-Driven Presses. Presses that have a final drive mechanism consisting of an eccentric or crank on the main shaft are the most widely used type of mechanical press. A connecting rod is used to convert the rotary motion of the main shaft into the reciprocating motion of the press ram. Generally, an adjustment mechanism is built into the connecting rod or press ram assembly, thus permitting the height position of the press ram to be changed with respect to the main shaft or press frame, thereby controlling the final pressing position of the ram. This adjustment mechanism can be used to control the length of the compacted part. Standard eccentric-driven presses have pressing capacities ranging from 6.7 to 7830 kN (0.75 to 880 tons).

Powder Metallurgy Presses and Tooling

Revised by John Porter, Cincinnati Incorporated

0 0

Post a comment