What is a Woodruff Key?
Woodruff key is semicircular shaped, such that, when installed, leaves a protruding tab. The keyway in the shaft is a semi-circular pocket, the mating part, a longitudinal slot. They are used to improve the concentricity of the shaft and the mating part, which is critical for high-speed operation.
The circular segment can be cut directly by plunge cutting with a circular Woodruff cutter without any reliefs.
This type of key was developed by William N. Woodruff of Hartford Connecticut. In 1888, he was awarded the John Scott Medal by the Franklin Institute for his invention. He came up with the idea so that a portion of the key protrudes. The key then prevents the part slipped over the shaft from freely rotating.
The more exact fit of the key and keyway also reduces play, and stress concentrations in, and improves the reliability of the key.
Woodruff key weakens the shaft, but there is little tendency for the key to turn over when in use.
Where lighter loads are transmitted and the cost of cutting a keyway is not justified, round keys and flat or hollow saddle keys can be used.
Woodruff keys are also relatively short and cannot carry the same load as longer keys. They are used for light-duty applications and mounting taper-bored components onto tapered shaft ends.
They’re inappropriate for transmitting heavy torque, such as with wide-faced gears or multi-groove V-belt pulleys. Mounting two or more Woodruff keys in line to carry higher loads is expensive and makes installation all the more problematic.
The main advantage of the Woodruff key is the elimination of milling near shaft shoulders, where stress concentrations and concentricity would be affected. The latter is particularly important for high-speed operation.
It also has the advantage that it will turn itself in its circular recess to accommodate any taper in the mating hub on assembly; for this reason, it cannot be used as a feather key since it would jam.
Woodruff keys are commonly used in machine tools and, for example, between the flywheel and the crankshaft of a small internal-combustion engine where the drive depends largely on the fit between the shaft and the conically bored flywheel hub.
An additional advantage is a stuck key can be removed from a shaft with a hammer blow, the circular profile will push the key out of the slot, as opposed to a standard key which will need to be pushed axially, or pulled out of its slot.
Common applications include machine tools, automotive applications, snowblower, and marine propellers.
Advantages of Woodruff Key
- It is an easily adjustable key.
- It is capable of accommodating any taper in the hub keyway,
- It’s a more exact fit of the key and the keyway also reduces play, and stress concentrations in, and improves the reliability of the key.
- It is eliminates milling near shaft shoulders, where stress concentrations and concentricity would be affected.
- It is particularly important for high-speed operation.
- This tapping is useful on shaft ends. Its extra depth in the shaft prevents any tendency to overturn it in its keyway.
Disadvantages of Woodruff key
- The depth of the keyway weakens the shaft.
- It cannot be used as a feather.
- Difficult to install and short, and
- It can’t carry too much load
Woodruff keys are semicircular shaped, such that, when installed, leave a protruding tab. The keyway in the shaft is a semi-circular pocket, the mating part, a longitudinal slot. They are used to improve the concentricity of the shaft and the mating part, which is critical for high-speed operation.
Semicircular Woodruff keys insert into a curved slot in the shaft. The advantage: Properly installed, it is captive and cannot walk out of the shaft. However, they are difficult to install. The key is first pressed into a curved slot milled in the shaft and then aligned so that its flat top is parallel with the shaft.
The main advantage of the Woodruff key is the elimination of milling near shaft shoulders, where stress concentrations, and concentricity would be affected. The latter is particularly important for high speed operation.
Woodruff Keys are used for locking gears and pulleys to shafts. On the shaft, they fit into a keyseat, which is a circular pocket that retains the key. An exposed portion of the key fits into a rectangular groove, often called the keyway, in the gear or pulley.