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Keys are made from drawn steel with a tensile strength of approximately 700 MPa. They must have a greater strength and hardness than the machine parts to be connected, so that they are not deformed when they are driven in.
Taper sunk keys
These are long bodies with a rectangular cross-section, inclined back surface and with plane or rounded front surface.
The inclination has the proportion of 1:100, which means: the taper is 1 mm per 100 mm.
- Round-ended sunk keys are called laid-in keys - they are pressed (or inserted) into the snugly fitting groove of the shaft and the hub is subsequently driven on to the sunk key.
These keys are used if there is no space for driving the key in of out- Straight-ended sunk keys, also called tapered driving keys - in this case the shaft and the hub (or the machine parts in question) are mounted as in normal use and the sunk key driven in subsequently.
They are used if there is sufficient space to drive them in and out from either side.- Tapered driving keys the thicker ends of which feature a nose are called gib-head keys.
They are used if driving in or out can be done from one side only.- Woodruff keys can also assume the function of taper sunk keys because - due to their rotatable mounting in the keyway they are able to adapt themselves to the taper in a hub keyway.
Figure 2 - Taper sunk keys
1 laid-in sunk key, 2 tapered driving key, 3 gib-head key, 4 Woodruff key
Hollow and flat keys
These are long bodies with a rectangular cross-section with inclined back surface and small taper. They are only used for transmitting little rotary forces. For these no keyway must be made:
- The bottom of hollow keys is concave in longitudinal direction. The edges of these keys resemble cutting edges which contact the shaft.- A good adaption of the flat key to the shaft is achieved only, if the shaft is flattened corresponding to the width of the key in that place where the key shall be applied.
Figure 3 - Hollow and flat
keys
Tangential keys
These consist in a pair of mating bodies of a rectangular cross-section. Each of these bodies has one inclined side face, the inclination proportion (taper) is 1:60 up to 1:100. Tangential keys are used if very great rotational forces have to be transmitted in both directions of rotation.
With their inclined surface turned towards each other, they are driven into inclined keyslots and hub keyways. In doing so, always two pairs of keys are staggered around the shaft circumference at an angle of 120°.
Figure 4 - Tangential keys
Taper sleeves
These are bodies in the form of truncated cones with internal
and external tapers serving to connect machine parts directly. In general they
are used with machine spindles where tools with taper shanks are applied. For
undoing the connection, cotters are driven through lateral oblong holes into the
taper sleeves. A special type of taper sleeve is the clamping sleeve, which, as
an intermediate, is used in machine part joints. Clamping sleeves are placed on
shafts on which then antifriction bearings, toothed gears and similar elements
can be mounted. Their uniform circumferential stress which is the result of a
taper between 1 in 10 and 1 in 20 guarantees exact true running.
They are
fastened by nuts.
Figure 5 - Taper sleeves and
clamping sleeves
Taper pins
These are elongated bodies in the form of truncated cones with a taper of 1 in 50.
In keyed joints they are also called ‘cylindrical taper keys’. They are used if a joint shall be made very simply, shall be undone only seldom and has to transmit only little rotational force, for instance levers on axles.
The bore holes are reamed by reamers as it is done with taper pin joints.
For undoing the joint the taper pin must be bored out.
Figure 6 - Taper pin
Cotters
These are rectangular bodies with one or two inclined surfaces the edges of which are rounded.
They are used for fixing bolts and crankshafts in order to transmit longitudinal (to-and-fro) movements.
Cotters have a taper of 1 in 10 up to 1 in 40 and are often secured against loosening by additional means.
Since the manufacturing of the slots requires much time and labour, they are used only, if great axial forces have to be transmitted.
For the transmission of little force, taper pins can be used instead, because this facilitates the making of the joint.
Figure 7 - Cotter
1 with one inclined surface
2 with two inclined surfaces
Tightening keys
These are rectangular bodies with one or two inclined surfaces and a tapped through hole in longitudinal direction.
Tightening keys transmit no rotary forces; they are used for clearance adjustment in divided bearings and guideways. They are applied across the rod axis. In order to achieve a great tightening effect by a short path of positioning in longitudinal direction, the back surfaces are manufactured with a taper between 1: 5 and 1:10.
Figure 8 - Tightening key
What are keyed
joints?
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What is the special advantage of keyed
joints?
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What disadvantage has to be considered with keyed joints at
rotating machine
parts?
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Which types of keys are used for joining machine parts that have
to carry out rotating
movements?
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What are the tasks cotters have to
fulfill?
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What are the tasks of tightening
keys?
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Hints on manufacturing keys
- One’s own keys are made only, if no industrially prefabricated ones are available. Keys are manually worked by filing and scraping.- Since the bottom and back surfaces require reworking when the key is fitted in, they are made with an allowance of 0.3 to 0.5 mm.
- Often, the length of the key can be exactly determined only when the connection is just being made. Therefore, a sufficiently long key should be prepared.
- The side faces of taper sunk keys get the h 9 fit, so that they have enough play in the groove.
Figure 9 - Surfaces of a key
1 top of key, 2 key bottom, 3 front surfaces, 4 side faces
Sequence of operations for making a key for a shaft-and-hub joint:
- Putting the hub on the shaft- Determining the key heights according to the dimensions of the grooves (reckoning the taper over again)
- Marking the material for the wedge
- Rough-finishing the taper by filing, breaking the comers 0.5 x 45°
- Fitting the key, finding out the drag marks or the bearing contact pattern, scribing the length of the key
- Finishing the surfaces
- Sawing the key to length, breaking the comers, deburring
What allowances have to be made when manufacturing a wedge
oneself?
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