A flat saddle key is a taper key which fits in a key way in the hub and is flat on the shaft as shown in Fig. 2. It is likely to slip round the shaft under load. Therefore it is used for comparatively light loads.
A hollow saddle key is a taper key which fits in a key way in the hub and the bottom of the key is shaped to fit the curved surface of the shaft. Since hollow saddle keys hold on by friction, therefore these are suitable for light loads. It is usually used as a temporary fastening in fixing and setting eccentrics, cams etc.
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1 saddle is a seat for a rider, usually made of leather, placed on a horse’s back and secured with a girth under the belly
2 saddle is a similar seat on a bicycle, tractor, etc., made of leather or steel
3 saddle is a back pad forming part of the harness of a packhorse
4 saddle is anything that resembles a saddle in shape, position, or function
5 saddle is a cut of meat, esp. mutton, consisting of part of the backbone and both loins
6 saddle is the part of a horse or similar animal on which a saddle is placed
7 saddle is the part of the back of a domestic chicken that is nearest to the tail
8 saddle (Civil engineering) is a block on top of one of the towers of a suspension bridge over which the cables or chains pass
9 saddle (Engineering) is the carriage that slides on the bed of a lathe and supports the slide rest, tool post, or turret
Reference A Textbook of Machine Design by R.S. Khurmi and J.K. Gupta