TYPES OF QUICK RETURN MECHANISM
There are three types of quick return mechanism
1. Hydraulic drive
2. Crank and slotted link mechanism
3. Whitworth mechanism
A quick return mechanism is a mechanism which produces a reciprocal effect so that the system takes less time in the return stroke while comparing with forward stroke. In the quick-return mechanism, a circular movement like the crank and lever mechanism is converted into reciprocal movement but the return time is different from the forward moment. In several applications, this process is used. Some of them are shaper, slotter, screw-press, mechanical drive, etc. The time required for cutting is reduced with the assistance of a quick return mechanism.
Inside the hydraulic cylinder a piston reciprocates. Between the piston and the ram a piston rod is connected. So, the ram reciprocates along with the piston. Two entries are provided near the each end of the cylinders. A four-way control valve connects these two entries with the reservoir which contains the fluid. The reservoir is connected to the valve with the help of a drain pipes and a supply pipe.
The supply pipe is again connected to the reservoir by a pump and relief valve. The valve is actuated by the lever and trip dog fitted to the ram. Oil is sucked by the gear pump from the reservoir at a particular pressure. This high pressure oil goes to the cylinder through the four-way valve.
The oil allowed from the pump to the left side of the piston which forces the piston to move the ram towards right (R). It is called as forward or cutting stroke. In this stroke, oil flows out on the right side entry to the reservoir through the four-way valve and drain pipe. The lever hits one trig dog at the end of this stroke.
Now, the lever position is changed. Due to this, the supply pipe supplies the oil on the right side of the piston which moves the ram towards left called as return stroke or nan-cutting stroke.in this stroke the high pressure oil covers on lesser area on the cylinder. Due to this, the pressure force will increase. Hence, this return stroke is faster by supplying the same quantity of oil.
CRANK AND SLOTTED LEVER MECHANISM
In this mechanism the ram is actuated by gear drives associated with electric motor. First, the electric motor drives the pinion gear. Next, the pinion gear drives the bull gear which rotates in opposite direction due to external gear meshing. A radial slide is provided on the bull gear. A sliding block is assembled on this slide. The block can be positioned in radial direction by rotating the stroke adjustment screw.
The sliding block has a crank pin. A rocker arm is freely fitted to this crank pin. The rocker arm sliding block slides in the slot provided in the rocker arm called as slotted link. The upper end has fork which is connected to the ram block by a pin while the bottom end of the rocker arm is pivoted.
When the pinion gear rotates along with the bull gear, the crank will also rotate. Due to this, the rocker arm sliding block also rotates in the same circle. Simultaneously, the sliding block slides up and down in the slot. This movement is transmitted to the ram which reciprocates. Hence, the rotary motion is converted in reciprocating motion.
WHITWORTH QUICK RETURN MECHANISM
The shaft of an electric motor drives the pinion which rotates the bull gear. The bull gear has a crank pinion. A sliding block slides over this crank pin and slides inside the slot of a crank plate. A connecting rod connects the pin at one end and ram at the other end.
When the pinion rotates, the bull gear is also rotated along with the crank pin. At the same time, the sliding block slides on the slot provided on the crank plate. This makes the ram to move up and down by the connecting rod.
The two important cases in this mechanism are
1. At an angle of the bull gear rotates in anticlockwise direction and the ram moves in forward direction.
2. When the bull gear rotates further in the same direction at an angle of the return stroke will take place. Here the angle is lesser than angle . So, the time taken for the return stroke is reduced.