Keys and Keyways
Keys are solid pieces of various shapes used in combination with mating, similarly shaped slots called keyways, to fasten two parts (usually to prevent relative circumferential or rotational motion to transmit torque). These mating fasteners may or may not prevent relative longitudinal or axial motion, depending on the type of key and keyway used. Figure 1 schematically illustrates how keys are used with keyways, while
Figure 2 schematically illustrates a variety of different key–keyway types. The designs and materials used for keys and for keyways or key seats are covered by ANSI B17.2 and B17.7. Materials used in keys are usually cold-finished steels, although other materials may be used for compatibility with the parts being fastened.
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| FIG 1 |
the function of keys in keyways for
(a) dovetail keys,
(b) beveled keys,
(c) round-tapered keys,
(d) flat-saddle keys,
(e) hollow-saddle keys,
(f) Woodruff keys.
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| FIG 2 |
STRENGTH OF KEYS
Figure 3.a represents a standard key joint with a radial clearance s between the key and the hub.
This clearance is quite large. For a 100-mm shaft, the nominal clearance is 0.4 mm, but taking into
account standard tolerances, it may come to 1 mm. Under load, the key turns under the action of
a couple of forces (from the shaft and the hub), which are not coaxial (Figure 3.b). Therefore,
the load distribution over the side surfaces of the key is highly uneven. If the radial clearance is
decreased to a minimum, the load distribution will be more uniform (Figure 3.c). But typical
tolerance values dictate the need for a certain clearance.
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| FIG 3 |
The bearing stress on the sides of the key is usually calculated under the assumption that the
entire torque is transmitted through the key, and the pressure is distributed evenly over the key’s
side surfaces:
(1)
where
T = torque
d = shaft diameter
l = length of the working surfaces of the key (without roundings)
h = height of the key (deducting chamfers)
the torque shall be transmitted mainly through the key between the hub and shaft. In addition, the load distribution over the height of the key obviously can’t be even. So this calculation is just a matter of convention.
The admissible stress is usually equal to the yield stress of the weakest material (shaft, hub, or key) divided by 2 or 3.
The reason for such a rough estimation of the bearing stress is that it is a hard task to determine
the actual load distribution on the key working surfaces. Besides, it is not so important because
some plastic deformation of the surfaces may lead to better load distribution. More important is
the shear stress calculation because, in this case, an overstress leads to failure of the joint (again,
assuming that the key is the only part that transmits torque). The shear stress equals:
(2)
where
b is the width of the key. Usually, b = (1.5–1.8)h, so if the bearing stress doesn’t exceed the allowable value, the shear stress is safely less than the allowable.
If it is desired that the hub move along the shaft, the admissible bearing stress calculated using
Equation (1) can be about 10 to 15 MPa. The key in this case should be attached to the shaft by
bolts to prevent its turning (Figure 3.d). Such a design is very archaic. Modern connections with
a movable hub are generally splined.
F=T/(d/2) (3)
where :
F : force on the key on each side
d : shaft diameter
The torque transmitted by the motor :
(4)
where :
P = motor power
N = rpm
Parallel keys and taper keys
acc to DIN 6885 Part 1 6886 and 6887
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