What Is A Woodruff Key?
A woodruff key is a half-moon-shaped machine key that prevents gears, hubs, or other components from moving independently of a rotating shaft or spindle. They also act as a fail‑safe that protects expensive parts by shearing under excessive loads. The crescent profile of the key allows it to sit deeper in the shaft reducing the likelihood of the key rolling laterally out of the groove.
The Woodruff key is a semi-circular disc and fits into a circular recess in the shaft which is machined by a woodruff keyway cutter. These woodruff keys are mostly used in machine tools and automobile shafts from ¼” to 2½” (6 mm to 60 mm) in diameter.
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. The main advantage of the Woodruff key is the elimination of milling near shaft shoulders, where stress concentrations and concentricity would be affected.
The more exact fit of the key and keyway also reduces play, and stress concentrations in, and improves the reliability of the key.
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 Woodruff key weakens the shaft, but there is little tendency for the key to turning 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.
Why Use A Woodruff Key?
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, snowblowers, and marine propellers.
How Does A Woodruff Key Work?
When installed, the long, flat face of the woodruff key protrudes above the host shaft. When a mating hub, gear, or other component is pressed onto the shaft, the protruding section of the key locks components together.
Installing A Woodruff Key: When installing a woodruff key, first press it into the semi‑circular groove milled into the shaft. Next, align the top, flat portion of the key so that it is parallel with the host shaft. During installation, woodruff keys will sometimes slip out of their groove as the mating component is pressed on. This can make woodruff keys difficult to install.