What is double slider-crank mechanism?

Short Answer:

A double slider-crank mechanism is a type of kinematic chain that consists of four links and two sliding pairs. It is a special form of the four-bar chain in which two of the turning pairs are replaced by sliding pairs. This mechanism converts rotary motion into reciprocating or oscillating motion and vice versa.

It is commonly used in machines where two sliding motions are required to produce rotary or oscillatory movement, such as in elliptical trammels, Scotch yoke mechanisms, and Oldham’s coupling. Each of these is an inversion of the double slider-crank mechanism obtained by fixing different links.

Detailed Explanation:

Double Slider-Crank Mechanism

A double slider-crank mechanism is one of the fundamental mechanisms in kinematics of machines. It is derived from a four-bar chain by replacing two of its turning pairs with sliding pairs. The mechanism consists of four links: a fixed frame, two sliders, and a connecting link that joins the sliders. It has two sliding pairs and two turning pairs.

This mechanism is highly useful in converting one type of motion into another, particularly rotary to reciprocating or oscillating motion, depending on which link is fixed. The name “double slider” comes from the presence of two sliding pairs in the chain.

Structure of Double Slider-Crank Mechanism

The double slider-crank mechanism consists of:

  1. Link 1 (Frame or Fixed Link): The stationary link that provides support and guides for the sliders.
  2. Link 2 and Link 4 (Sliders): These are the sliding members that move back and forth in the slots or guides of the frame.
  3. Link 3 (Connecting Link): It connects the two sliders through turning pairs and transfers motion between them.

The arrangement allows the sliders to move linearly in perpendicular directions while the connecting link moves in a circular or elliptical path.

Working Principle

The basic principle of the double slider-crank mechanism is the conversion of one form of motion to another using two sliding pairs. When one slider is moved or rotated, the connecting link transmits motion to the other slider, producing either a linear, rotary, or oscillating movement depending on which link is kept fixed.

The mechanism can be used for generating complex paths, such as ellipses, or for transmitting motion between misaligned shafts. The relative motion between the sliders and connecting link determines the nature of the output motion.

Inversions of Double Slider-Crank Mechanism

There are three important inversions of the double slider-crank mechanism obtained by fixing different links:

  1. Elliptical Trammel (First Inversion)

In this inversion, the frame (link 1) is fixed. Two slots are provided in the frame, placed at right angles to each other. Two sliders move within these slots and are connected by a rigid link. The midpoint of the connecting link traces an elliptical path during motion.

Applications:

  • Used as an elliptical trammel or trammel of Archimedes, a device used to draw ellipses.
  • Also used in some measuring instruments for geometric tracing.

Working:
As one slider moves horizontally and the other vertically, the connecting link’s center moves along an ellipse, demonstrating how combined linear motions produce curved motion.

  1. Scotch Yoke Mechanism (Second Inversion)

In the second inversion, the connecting link (link 3) is fixed. One of the sliders moves in a slotted frame attached to the crank, and the other slider moves in a straight guide. The rotary motion of the crank is converted into a reciprocating motion of the slider.

Applications:

  • Used in reciprocating engines and pumps for smooth transmission of motion.
  • Also used in small air compressors and mechanical actuators.

Working:
As the crank rotates, the pin fixed to it slides inside the slot of the connecting piece. This sliding action causes the slider to move back and forth in a straight line, thus converting rotary motion into reciprocating motion.

  1. Oldham’s Coupling (Third Inversion)

In this inversion, one of the sliders (link 2 or 4) is fixed. It is used to connect two parallel shafts that have a small lateral misalignment. The coupling consists of three discs — one attached to each shaft and the third (middle disc) that slides between them with slots at right angles to each other.

Applications:

  • Used in machine tools, automotive systems, and rotating machinery to connect misaligned shafts.

Working:
When one shaft rotates, the middle disc slides and transmits motion to the other shaft, compensating for misalignment while maintaining equal angular velocity between the two shafts.

Importance and Applications

The double slider-crank mechanism is widely used because of its ability to convert and transmit motion efficiently.

  • It allows conversion between linear and rotary motion.
  • It provides accurate path generation, such as ellipses or reciprocating strokes.
  • It is used in mechanical linkages, engines, pumps, couplings, and drawing instruments.

This mechanism’s versatility makes it a key component in mechanical design and automation systems.

Conclusion

The double slider-crank mechanism is an important kinematic chain in mechanical engineering that demonstrates how sliding and turning pairs can work together to generate different types of motion. By fixing different links, various useful mechanisms like the elliptical trammel, Scotch yoke, and Oldham’s coupling are obtained. These inversions are used in numerous machines for converting motion types and ensuring smooth power transmission. Understanding this mechanism helps engineers design efficient and reliable motion systems for industrial and mechanical applications.