Chapter 11 keys, coupling, and seals

11-2 Keys

A key is a machinery component placed at the interface between a shaft and the hub of a power-transmitting element for the purpose of transmitting torque

Square and Rectangular Parallel Keys

The most common type of key for shafts up to 6.5 inches in diameter is the square key. The rectangular key is recommended for large shafts and is used for smaller shafts where the shorter height can be tolerated. Both square and the rectangular keys are referred to as parallel keys because the top and bottom and the sides of the key are parallel

The keyseats in the shaft and the hub are designed so that exactly one-half of the height of the key is bearing on the side of the shaft keyseat and the other half on the side of the hub keyseat.

GB 1096-79 普通平键联接

楔键联接

Taper keys and Gib head key Taper keys are designed to be inserted from

the end of the shaft after the hub is in position rather than installing the key first and then sliding the hub over the key as with parallel keys.

The gib head key has a tapered geometry inside the hub that is the same as that of the plain taper key. But the extended head provides the means of extracting the key from the same end at which it was installed. This is very desirable if the opposite end is not accessible to drive the key out.

Pin keys The pin key is a cylindrical

pin placed in a cylindrical groove in the shaft and hub. Lower stress concentration factors result from this design as compared with parallel or taper keys. A close fit between the pin and the groove is required to ensure that the pin does not move and that the bearing is uniform along the length of the pin.

Woodruff Keys

Where light loading and relatively easy assembly and disassembly are desired, the Woodruff key should be considered.

Selection and Installation of Keys and Keyseats

The key and the keyseat for a particular application are usually designed after the shaft diameter is specified. Then, with the shaft diameter as a guide, the size of the key is selected from standard data. The only remaining variables are the length of the key and its material, One of these can be specified, and the requirements for the other can then be computed.

The length of a key Typically the length of a key is

specified to be a substantial portion of the hub length of the element in which it is installed to provide for good alignment and stable operation. But if the keyseat in the shaft is to be in the vicinity of other geometric changes, such shoulder fillets and ring grooves, it is important to provide some axial clearance between them so that the effects of the stress concentrations are not compounded.

11-3 Materials for keys Keys are most often made from low-

carbon, cold-drawn steel (AISI 1020 is commonly used ). Standard key is available from industrial supply centers in similar materials. You should check the actual material and guaranteed strength of key stock used in critical application.

If the low-carbon steel is not sufficiently strong, higher-carbon steel, such as AISI1040 or 1045, could be used.

11-4 Stress analysis to determine key length

There are two basic modes of potential failure for keys transmitting power: (1) shear across the shaft/hub interface and (2) compression failure due to the bearing action between the sides of the key and the shaft or hub material. The analysis for either failure mode requires an understanding of the forces that act on the key.

Strength of key connection

Shear strengthThe magnitude of the shearing force F=T/(D/2)The shear stress τ=F/A=2T/DWLAllowable shear stress τd=0.5Sy/N (N---design factor

N=3 is typical industrial applications))So L=2T/ τdDW

Contact stress

Allowable stress of compression σd=Sy/N

The compress stress σ=4T/DLHRequire length of key L=4TN/DWSy

Allowable stress of key material

Example problem 11-1

A portion of a shaft where a gear is to be mounted is to be 2.00 in. The gear transmits 2,965lb.in of torque. The shaft is to be made of AISI 1040 cold-drawn steel. The gear is made from AISI 8650 OQT 1000 steel. The width of the hub of the gear mounted at this location is 1.75 in. Design the key.