Quick Return Mechanism

QUICK RETURN MECHANISM

INTRODUCTION: 

A quick return mechanism is an apparatus to produce a reciprocating motion in which the time taken for travel in the return stroke is less than in the forward stroke. It is driven by a circular motion source and uses a system of links with three turning pair and one sliding pair. This mechanism is mostly used in shaping machines, slotting machines and in rotary internal combustion engines. 

 

TYPES OF QUICK RETURN MECHANISM:

1. Whitworth Quick Return Mechanism  

2. Crank and Slotted Link Mechanism 

3. Hydraulic Drive 

 

WHITWORTH QUICK RETURN MECHANISM:

Fig 1: WHITWORTH QUICK RETURN MECHANISM

This mechanism converts rotary motion into oscillating motion, just like chronic and lever mechanism. Whitworth's quick return system is the second inversion of the slider crank mechanism in which the slider is fixed. In this mechanism, the bull gear is used and it has a crank pinion. The sliding block slides over the crankpin and slides inside the slot of a crank plate and the connecting rod connects the pin at one end ram at the other end.

The electric motor shaft drives the pinion and rotates the gear. Now the bull gear is also rotating along with the crankpin and the sliding block slides on the slots provided on the crank plates. So the process makes the ram move forward and backward. This mechanism has a return stroke or, ideally, faster than the forward stroke. 

This mechanism is mostly used in shaping and slotting machines. In the mechanism, the link CD (link 2) forming the turning pair is fixed, as shown in Fig. Link 2 corresponds to crank in a reciprocating steam engine. The driving crank CA (link3) rotates at a uniform angular speed. The slider (link 4) attached to the crank pin at A slides along the slotted bar PA (link 1). Which oscillates at a pivoted point D. The connecting rod PR carries the ram at R to which a cutting tool is fixed. The motion of the tool is constrained along the line RD produced, i.e. along a line passing through D and perpendicular to CD. 

When the driving crank CA moves from the position DP1 to DP2 through an angle ⍺ in the clockwise direction, the tool moves from the left-hand end of its stroke to the right-hand end through a distance 2PD. 

Now when the driving crank moves from the position CA2 toCA1 (or the link DP from DP2 to DP1) through an angle β in the clockwise direction, the tool moves back from the right-hand end of its stroke to the left-hand end.

It is seen that the time taken during the left to the right movement of the ram (i.e. during forward or cutting stroke) will be equal to the time taken by the driving crank to move fromCA1 to CA2. Similarly, the time taken during the right to left movement of the ram (or during the idle or return stroke) will be equal to the time taken by the driving crank to move CA2 to CA1.

Since the crank link CA rotates at uniform angular velocity therefore time taken during the cutting stroke (or forward stroke) is more than the time taken during the return stroke. In other words, the mean speed of the ram during cutting stroke is less than the mean speed during the return stroke.

CRANK AND SLOTTED LEVER QUICK RETURN MOTION MECHANISM:

Fig 2: CRANK AND SLOTTED LEVER QUICK RETURN MOTION MECHANISM

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 center C. A sliding block is 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 f stroke R1R2. The line of stroke of the ram (i.e. R1R2) is perpendicular to AC produced. 

In the extreme positions, AP1 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 ꞵ) in the clockwise direction. The return stroke occurs when the crank rotates from the position CB1 to CB2 (or through angle 𝛼) 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. 

HYDRAULIC DRIVE:

Fig 3: HYDRAULIC DRIVE

The hydraulic drive mechanism is one of the mechanism used in shaper machines. In this mechanism, the ram is moved back and forth by a piston rotating in a cylinder placed under the ram. This machine consists of a continuous discharge oil pump, a cylinder, a valve chamber, and a piston. The piston ram is fastened to the ram body. Hydraulic fluid is used in the hydraulic quick return mechanism to speed the ram.

In hydraulic drives, the bottom consists of a tank that holds hydraulic fluids. This tank is also known as an oil reservoir. Earlier oil used to come out of the reservoirs. This oil is passed through the valve chambers to the right of the oil cylinder, pressurizing the piston is discharged into the reservoir through the throttle valve. First, the fluid in the tanks is pumped out, and this passage on the right side of the cylinder. This fluid passes through the passage on the right side of the cylinder. This fluids exerts pressure on the pistons, and the machine mess strokes forward. 

When the ram moves forwards, the lever changes its positions and kills the overturned dog. As the lever changes its positions, the three valves attached to the lever also change their position, and the oil can now pass through the passage on the left side of the cylinder. After the completion of the forward stroke, the valves change their position, and the pump fluid from the reservoir now runs along the passage to the left of the piston. Also, the pass-through, which oil returns to the reservoir, opens and loins the right path, and the fluid on the right side of the piston reaches the reservoir. 

As the fluid moves to the left of the piston, the piston that attaches to the ram moves to the right, and a return stroke is made by the ram. At the end of the return strokes, another dog was hit against the lever, and the stroke changed along the direction of the lever. In this way, the forward and return strokes of ram is repeated. a quick return occurs due to the difference in stroke volume of the cylinder at the both ends. The volume of the cylinder at the left hand side less than the amount of the right hand passage.

As the pump is a continuous discharge pump, the same amount of oil will be passed on both routes. So with lower volume, the pressure n pass will be greater, and the return stroke will be fasters than the forward stroke. The cuttings speed can be controlled by controlling the flow of oil, which can be controlled using the throttle valve. When the throttle valve is lost, the additional valve is cut through the relief valve, maintaining a uniform pressure during the cutting stroke.

 

ADVANTAGES:

1. The process is automated.

2. The construction of the mechanism is not so complicated.

3. It can perform operations like cutting, flattening and slotting the workpiece.

4. The idle time is reduced because of the fast returning stroke.

 

DISADVANTAGES:

1. In the returning stroke, there is no contact with the work so no cutting takes place hence the process takes much time to complete.

2. The forward stroke takes much time compared to the return stroke. 

3. It takes more power to perform operation.

4. There is friction in the slider and piston.

5. Continuous it won't work because of heat generated inside the piston and wear and tear can happen.

6. Balancing of linkage is also a major problem as this device is also linked to linkages.

 

CONCLUSION:

The quick return mechanism is known as a principle used to converts circular motion into reciprocating motion, allowing the slider to move forward and backward. The cutting process occurs in the forward motion, but there is no corresponding cutting in the reverse direction. That is all for this article, where the definition, applications, diagram, parts, types, working principle, advantages, disadvantages of quick return mechanism are being explained.