QUICK RETURN 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.

 

There are three types of quick return mechanism:

1. Hydraulic drive

2. Crank and slotted link mechanism

3. Whitworth mechanism

 

1) HYDRAULIC DRIVE:

Basic Parts of Hydraulic Drive:

Ø  Base

Ø  Piston

Ø  Pump

Ø  Control Valve

Ø  Relief Valve

Ø  Throttle Valve

Ø  Ram

Ø  Tool Head

Ø  Motor

  In hydraulic drive, there is a tank at the bottom which contains the hydraulic fluid. This tank is also known as oil reservoir.

This oil is passed through the valve chamber present in the right of the oil cylinder exerting pressure on the piston.

Any oil present in the left side of the piston is discharged to the reservoir through the throttle valve.

At first the fluid in the tank is pumped out and this fluid passes through the passage present in the right side of the cylinder .

This fluid exerts pressure on the piston and the ram of the machine performs forward stroke.

When the ram moves forward, the lever changes its position and hits the reversing dog. As the lever changes its position, the three valves connected to the lever also change their position and now the oil can pass through the passage present in the left side of the cylinder.
After the forward stroke is completed, the valves changes its position and now the pumped fluid from the reservoir moves from the passage present in the left side of the piston. Also, the passage through which the oil return to the reservoir opens and get connected to the right passage and the fluid present on the right side of the piston is discharge to the reservoir.
As the fluid moves towards the left side of the piston, the piston which is attached to the ram moves towards right and return stroke is performed by the ram.
At the end of the return stroke, another dog hit against the lever and the direction of the lever as well as the stroke changes. In this way, the forward and the return stroke of the ram is repeated.
The quick return takes place due to difference in the stroke volume of cylinder at both ends. The volume of passage at the left side is less than the volume of the passage on the right side. As the pump is constant discharge pump, same amount of oil will be passed on the both passage. So the pressure n the passage with less volume will be more and the return stroke will be faster than the forward stroke.

The cutting speed can be controlled by controlling the flow of oil which can be controlled by using the throttle valve.

When the throttle valve is lost the excess valve is cut through the relief valve to the reservoir maintaining uniform pressure during the cutting stroke.

       ADVANTAGES:

Ø  Hydraulic drive has lower first cost

Ø  It is much simpler in operation

Ø  It does not make any noise and operates very quietly.

Ø  Ability to stall against an obstruction without damage to the tool.

Ø  Ability to change length and position of stroke or speed while the machine is running.

Ø  The cutting and return speeds are practically constant throughout the stroke. This permits the cutting tool to work uniformly during cutting stroke.

Ø  The reversal of the ram is obtained quickly without any shock as the oil on the other end of the cylinder provides cushioning effect.

Ø  It offers great flexibility of speed and feed.

Ø  The hydraulic drive shows a very nearly constant velocity as co which has a constantly changing velocity because the horizontal component of the crankpin moving about its circle is constantly changing.

DISADVANTAGES :

Ø   Handling hydraulic fluids is messy

Ø   It can be difficult to totally get rid of leaks in a hydraulic system. If hydraulic fluid leaks in hot areas, it may catch fire. If hydraulic lines burst, they can cause serious injuries. Take care when handling hydraulic fluids, as too much exposure can lead to health issues.

Ø  Hydraulic fluids are also corrosive.

Ø  To keep your hydraulic system in its prime, periodically check hydraulic systems for leaks, lubricate when necessary, and change filters and seals as required. Ie need more cost for maintainance and repair.

2) CRANK AND SLOTTED LEVER MECHANISM:

The crank and slotted quick return mechanism converts rotatory motion into linear motion. It is extensively used in shaping and cutting machines and is particularly useful in cutting flat surfaces out of metal stock.The crank and slotted quick return mechanism was invented by Whitworth in the 1800s.

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.

 This mechanism is mostly used in shaping machines, slotting machines and in rotary internal combustion engines. In this mechanism, the link AC (i.e. link 3) forming the turning pair is fixed, as shown in fig. The link 3 corresponds to the connecting rod of a reciprocating steam engine. The driving crank CB revolves with uniform angular speed about the fixed centre C. A sliding block attached to the crank pin at B slides along the slotted bar AP and thus causes AP to oscillate about the pivoted point A. A short link PR transmits the motion from AP to the ram which carries the tool and reciprocates along the line of stroke R1R2. The line of stroke of the ram (i.e. R1R2) is perpendicular to AC produced.

In the extreme positions, API and AP2 are tangential to the circle and the cutting tool is at the end of the stroke. The forward or cutting stroke occurs when the crank rotates from the position CB1 to CB2 (or through an angle β) in the clockwise direction. The return stroke occurs when the crank rotates from the position CB2 to CB1 (or through angle α ) in the clockwise direction. Since the crank has uniform angular speed.

therefore,

  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.

3) WHITWORTH QUICK RETURN MECHANISM:

The 'Whitworth' quick return mechanism converts rotary motion into reciprocating motion, but unlike the crank and slider, the forward reciprocating motion is at a different rate to the backward stroke. At the bottom of the slotted arm, the peg only has to move through a few degrees to sweep the arm from left to right, but it takes the remainder of the revolution to bring the arm back. This mechanism is most commonly seen as the drive for a shaping machine.

The circular rotating disk is called the Crank. o The link connected to the rotating disk is called Slider.The crank is also the input drive, it gives the rotational motion and the slider results in rocker motion, which further results in the motion of the 3rd linkage i.e.to and fro motion. Basic differences between crank and slotted lever mechanisms and a Whitworth quick return mechanism is the path of return.  In whitworth mechanism, the forward travel is different arc, and returning is a different arc( a smaller circle). Whereas in crank and slot mechanism, the forward and backward, both are via a same arc hence take same time to traverse.  Whittworth is usually used where arc / path of return is usually not fruitful. Like, where angle of blade cutting in only one direction. In another direction, it just had to return to original point, without doing any cutting.

Application of Whitworth Mechanism-

Shaping Machine A quick return mechanism is used where there is a need to convert rotary motion into reciprocating motion. As the disc rotates the black slide moves forwards and backwards. Many machines have this type of mechanism shaping machine is the best example.Shaping Machine A shaper is a type of machine tool that uses linear relative motion between the work piece and a single-point cutting tool to machine a linear tool path. It’s a machine that can make slots and grooves using a thin cutting tool.The linear cutting motion is provided by the “Quick- Return Mechanism“.

Published by-

1) AAYUSH SURAWAR

2) SAMARTH TAKBHATE

3) DEVASHISH TAMBADE

4) UMESH WANARE

5) CHINTAN VORA