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.
How To Keep a Woodruff Key in Place?
To keep a woodruff key in place while installing a mating component, ensure that it is situated tightly in the key seat. Since woodruff keys are made to tight tolerances, they often require a press-fit into its seat.
This is done by lightly tapping the flat, protruding face of the key with a hammer or mallet until the key is seated firmly. The key will still extend above the mating shaft’s surface to lock the mating hub or gear in place. If the key still falls or slips out during component installation, the key is likely too small for the host groove.
Selecting The Right Woodruff Key
It is important to understand the different features of a woodruff key during the design phase of an application. Some considerations include key type, key material, and key finish.
What Are the Different Types of Woodruff Keys?
The two most common woodruff keys are the full radius and a flat bottom. Both function the same but may be appropriate for different applications. It is important to note that both radius and flat bottom woodruff keys are standard and either is shipped at the manufacturer’s discretion. If a specific type is required,
- Full radius woodruff keys are a full half circle and make full contact with the groove seat. Full radius woodruff keys require precision of the seat groove as contact can occur between the woodruff key and the bottom of the seat causing interference and the key to protrude from the groove higher than desired.
- Flat bottom woodruff keys have the same profile as standard woodruff keys, however, the bottom of the key has been shaved flat. The flat bottom reduces the potential of interference at the bottom of the seat, allowing the woodruff key to seat fully in the groove and alleviating potential clearance issues of excessive key height.
- Hi‑Pro keys are a variant of woodruff keys that feature a full radius but have the added feature of feet. When installed, the feet extend over the face of the host shaft, preventing the key from rocking in its rounded seat.
Woodruff Key Materials
Woodruff keys are crafted in carbon, alloy, and stainless steel. Carbon and alloy steel are both durable, however, they must be plated or coated to prevent corrosion. Stainless steel surpasses carbon and alloy steel in corrosion resistance, but it is not as strong.
Woodruff Key Finishes
There are multiple finishes available for woodruff keys. Finishes include plain, clear, and yellow zinc, and passivated.
- Plain finish is available on carbon steel and alloy steel woodruff keys. This finish is an economical solution for keys that will not be subjected to corrosive elements.
- Zinc plated woodruff keys are plated in either yellow or clear zinc on carbon steel keys. The zinc plating offers an increased level of corrosion resistance compared to plain parts.
- Passivated finish is an oxidation process for stainless steel parts. This microfilm is added to provide additional corrosion resistance to the already resilient material.
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