Double Slider Crank Mechanism

Definition

A double slider crank mechanism is a kinematic chain consisting of four links, specifically designed with two turning pairs and two sliding pairs. It is an inversion of the four-bar chain mechanism, where two of the links are sliders that move within designated tracks or slots.

Main Content

1. Inversion Principle

• The mechanism is derived from the "Double Slider Crank Chain," which is a kinematic chain having two turning pairs and two sliding pairs.
• By fixing different links of this chain, we obtain various important mechanisms used in engineering, such as the Elliptical Trammel or the Scotch Yoke mechanism.

2. The Four-Bar Chain Relationship

• It is a modification of the standard four-bar linkage where sliding joints replace some of the rotating pins.
• The mobility of the mechanism remains consistent with Grashof's Law, ensuring that the sliders perform predictable oscillating or reciprocating motions.

3. Structural Configuration

• The mechanism typically consists of a frame (the fixed link), two sliders, and a connecting link (often called the intermediate link or the bar).
• The geometry allows for the conversion of rotary motion into reciprocating motion or vice versa, depending on the inversion.

       [Slider 1]
           |
    [Link AB]----- [Slider 2]
           |
       [Slot/Frame]

Working / Process

1. Input Motion Initiation

• The process begins when an external force is applied to one of the sliders or the rotating crank.
• The input motion defines whether the system acts as a generator of harmonic motion or as a geometric tool.

2. Intermediate Transmission

• The link connecting the two sliders acts as the transmission member.
• As one slider moves in a horizontal plane, the connecting link forces the second slider to move in a vertical plane (or a path perpendicular to the first), depending on the design layout.

3. Output Translation

• The output is the resulting motion of the second slider.
• In mechanisms like the Scotch Yoke, this output is a pure simple harmonic motion, which is crucial for precision instruments and pumps.

Advantages / Applications

• Elliptical Trammels: Used to draw precise ellipses by allowing two sliders to move in perpendicular slots.
• Scotch Yoke Mechanism: Highly efficient for converting rotational motion into linear reciprocating motion, commonly used in control valves and high-pressure pumps.
• Oldham’s Coupling: Used to connect two parallel shafts whose axes are at a small distance apart, allowing for smooth torque transmission despite minor misalignment.

Summary

The double slider crank mechanism is a four-link kinematic chain featuring two turning and two sliding pairs, functioning primarily to convert or guide reciprocating and rotary motion. Key concepts include kinematic inversion, simple harmonic motion, and mechanical displacement. Important terms to remember: Kinematic Chain, Sliding Pair, Turning Pair, and Inversion.