physics

Leg mechanism

Leg mechanism is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Leg mechanism rather than just read about it. In short: A leg mechanism (walking mechanism) is a mechanical system designed to provide a propulsive force by intermittent frictional contact with the ground. This is in contrast with wheels or continuous tracks which are intended to maintain continuous frictional contact with the ground.

Key takeaways

Leg mechanism belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Leg mechanism to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Leg mechanism from memory before moving on to harder problems.

Reference excerpt

A leg mechanism (walking mechanism) is a mechanical system designed to provide a propulsive force by intermittent frictional contact with the ground. This is in contrast with wheels or continuous tracks which are intended to maintain continuous frictional contact with the ground. Mechanical legs are linkages that can have one or more actuators, and can perform simple planar or complex motion. Compared to a wheel, a leg mechanism is potentially better fitted to uneven terrain, as it can step over obstacles. An early design for a leg mechanism called the Plantigrade Machine by Pafnuty Chebyshev was shown at the Exposition Universelle (1878). The original engravings for this leg mechanism are available. The design of the leg mechanism for the Ohio State Adaptive Suspension Vehicle (ASV) is presented in the 1988 book Machines that Walk. In 1996, W-B. Shieh presented a design methodology for leg mechanisms. The artwork of Theo Jansen, see Jansen's linkage, has been particularly inspiring for the design of leg mechanisms, as well as the Klann patent, which is the basis for the leg mechanism of the Mondo Spider.

Design goals horizontal speed as constant as possible while touching the ground (support phase) while the foot is not touching the ground, it should move as fast as possible constant torque/force input (or at least no extreme spikes/changes) stride height (enough for clearance, not too much to conserve energy) the foot has to touch the ground for at least half of the cycle for a two/four leg mechanism or respectively, a third of the cycle for a three/six leg mechanism minimized moving mass vertical center of mass always inside the base of support the speed of each leg or group of legs should be separately controllable for steering the leg mechanism should allow forward and backward walking Another design goal can be, that stride height and length etc. can be controlled by the operator. This can relatively easily be achieved with a hydraulic leg mechanism, but is not practicable with a crank-based leg mechanism. The optimization has to be done for the whole vehicle – ideally the force/torque variation during a rotation should cancel each other out.

History Richard Lovell Edgeworth tried in 1770 to construct a machine he called a "Wooden Horse", but was not successful.

Patents Patents for leg mechanism designs range from rotating cranks to four-bar and six-bar linkages. See for example the following patents:

U.S. Patent No. 469,169 Figure Toy, F. O. Norton (1892). U.S. Patent No. 1,146,700, Animated Toy, A. Gund (1915). A leg mechanism formed by an inverted slider-crank. U.S. Patent No. 1,363,460, Walking Toy, J. A. Ekelund (1920). A leg mechanism formed by a rotating crank with extensions that contact the ground. U.S. Patent No. 1,576,956, Quadruped Walking Mechanism, E. Dunshee (1926). A four-bar leg mechanism that shows the coupler curve forms the foot trajectory. U.S. Patent No. 1,803,197, Walking Toy, P. C. Marie (1931). Another rotating crank leg mechanism. U.S. Patent No. 1,819,029, Mechanical Toy Horse, J. St. C. King (1931). A crank-rocker leg mechanism with a one-way friction mechanisms in the foot. U.S. Patent No. 2,591,469, Animated Mechanical Toy, H. Saito (1952). An inverted slider crank mechanism for the front foot and crank-rocker for the back foot. U.S. Patent No. 4095661, Walking Work Vehicle, J. R. Sturges (1978). A lambda mechanism combined with a parallelogram linkage to form a translating leg that follows the coupler curve. U.S. Patent No. 6,260,862, Walking Device, J. C. Klann (2001). The coupler curve of a four-bar linkage guides the lower link of an RR serial chain to form a leg mechanism, known as the Klann linkage. U.S. Patent No. 6,481,513, Single Actuator per Leg Robotic Hexapod, M. Buehler et al. (2002). A leg mechanism that consists of a single rotating crank. U.S. Patent No. 6,488,560, Walking Apparatus, Y. Nishikawa (2002). Another rotating crank leg mechanism.

Gallery

Stationary

Walking

Complex mechanism Shown above are only planar mechanisms, but there are also more complex mechanisms:

See also Hexapod (robotics) Jansen's linkage Kinematics Kinematic pairs Klann linkage Chebyshev's Lambda Mechanism Linkage (mechanical) Machine Mecha Mobile robot

References

External links

Media related to Leg mechanism at Wikimedia Commons

Illustrations

Worked examples

Example 1 — a first encounter with Leg mechanism

Start with the simplest possible case. Write down what Leg mechanism claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Leg mechanism before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Leg mechanism ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Leg mechanism

In research: Leg mechanism appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Leg mechanism in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Leg mechanism is common in secondary-school and first-year university syllabi. It links to neighbouring topics Linkages (mechanical), so understanding it makes those chapters shorter.
In everyday life: Look for Leg mechanism outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Leg mechanism in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Leg mechanism means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Leg mechanism out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Leg mechanism in simple terms?

Why does Leg mechanism matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Leg mechanism?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Leg mechanism.