Machine Elements: Keys and Pins

In mechanical engineering, a key is a machine element used to connect a rotating machine element to a shaft. Through this connection the key prevents relative rotation between the two parts and allows torque to be transmitted through. For a key to function the shaft and rotating machine element must have a keyway, also known as a keyseat, which is a slot or pocket for the key to fit in. The whole system is called a keyed joint. There are three main types of keys: parallel, Woodruff, and tapered keys.

The keyway in a shaft for a parallel key A sprocket with an internal parallel keyway Cross-section of a parallel keyed joint

Parallel keys are the most widely used. They have a square or rectangular cross-section. Square keys are used for smaller shafts and rectangular faced keys are used for shaft diameters over 6.5 in (170 mm) or when the wall thickness of the mating hub is of concern. The keyway is a longitudinal slot in both the shaft and mating part.

Fig 1.
Fig 2.
Fig 3.
Fig 4

Woodruff keys are semicircular shaped keys that, when installed, leave a protruding tab. The keyway in the shaft is a semi-circular pocket and the mating part has a longitudinal slot. They are used to improve the concentricity of the shaft and the mating part, which is critical for high speed operations. The main advantage of the Woodruff key is that avoids the milling of a keyway near shaft shoulders, which already have stress concentration. Common applications include machine tools, automotive applications, snowblowers and marine propellers. In figure 1 you can see gear G that is held on shaft S by Woodruff key N.

The final type of key is the tapered key, which is tapered on one side, the side that engages the hub. The keyway in the hub is broached with a taper matching that of the tapered key. Some taper keys have a gib, or tab, for easier removal during disassembly. The problem with taper keys is that they can change the center of rotation of the shaft to be slightly off of the mating part. A pin is a device used for fastening objects or material together. It is usually made of steel, or on occasion copper or brass. Tapered pins (Fig. 2) can be used to transmit very small torques or for positioning. They should be fitted so that the parts are drawn together to prevent their working loose when the pin is driven home. Straight pins, likewise, are used for transmission of light torques or for positioning. Spring pins have come into wide use recently. Two types shown in Fig. 3 deform elastically in the radial direction when driven. They can replace straight and taper pins and combine the advantages of both, i.e., simple tooling, ease of removal, reusability, ability to be driven from either side. Cotter pins (Fig. 4) are used to secure or lock nuts, clevises, etc. Driven into holes in the shaft, the eye prevents complete passage, and the split ends, deformed after insertion, prevent withdrawal.


keyseat or keyway – шпонковий паз / канавка

feather or spline key – шпонка напрямна або призматична

according to – згідно

in most cases – у більшості випадків

dovetailed key – шпонка у вигляді ластівкового хвосту

slot – паз

shaft – вал

in spite of – не зважаючи на

mating part – парна деталь

positioning – розміщення

insertion – вставка, вкладка


Task 3. Answer the following questions:

1. What is the main function of a key?

2. What types of keys can you name?

3. How can keys be distinguished according to various characteristics?

4. What is the difference between cotter and tapered pins?

5. How can pins be divided geometrically?

6. What is the form of the keyway in the shaft if we are talking of a Woodruff key?

7. What are the advantages of spring pins?

8. What is the main disadvantage of tapered pins?


Task 4. Find English equivalents for the following words and word combinations in the text:

шпонка напрямна, обертальний рух, конічна шпонка, крутний момент, шпонка у вигляді ластівкового хвоста, не звертаючи увагу на, вісь, гайка, передача, перевага, застосування, з’єднання, встановити, навантаження, поперечний переріз, скоба, парна деталь.

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