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Untitled Prezi

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mahmoud abdulaziz

on 18 May 2015

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How to connect rotating mechanical elements with shafts


Some types of mechanical elements used to connect rotating elements with shafts :
keys flange splines
pins couplings
Key definition:

A key is a piece of steel inserted between the shaft and hub or boss of the pulley to connect these together in order to prevent relative motion between them. It is always inserted parallel to the axis of the shaft. Keys are used as temporary fastenings and are subjected to considerable crushing and shearing stresses.
A keyway is a slot or recess in a shaft and hub of the pulley to accommodate a key.

The Function of Keys:

-There are two main functions of keys when they are used to lock the transmission shaft.

-The primary function is to transfer torque from the rotating shaft to the rotary element of the machine. The same key works to transmit torque in both direction; from shaft to the machine element, or from machine element to the shaft.

-The second function of the key is to
restrict relative rotational motion and axial movement between the shaft and the machine element. Some special types of keys are also available, such as feather and splines, keys that allow axial movement between them.

Types of keys:
Sunk keys:
The sunk keys are provided half in the keyway of the shaft and half in the keyway of the hub or boss of the pulley or gear.
Saddle keys:
The saddle keys are of the following two types :
(a) Flat saddle key. (b) Hollow saddle key.

A flat saddle: key is a taper key which
fits in a keyway in the hub and is flat on the shaft.
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 keyway 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.

Tangent Keys :
The tangent keys are fitted in pair at
right angles. Each key is to withstand torsion in one direction only. These are used in large heavy duty shafts.

Round Keys :
The round keys, are circular in section and fit into holes drilled partly in the shaft and partly in the hub. They have the advantage of manufacturing as their keyways may be drilled and reamed after the mating parts have been assembled. Round keys are usually considered to be most appropriate for low power drives.

Splines :
Sometimes, keys are made integral with the shaft which fit in the keyways broached in the hub. Such shafts are known as splined shafts. These shafts usually have four, six, ten or sixteen splines. The splined shafts are relatively stronger than shafts having a single keyway.
The splined shafts are used when the force to be transmitted is large in proportion to the size of the shaft as in automobile transmission and sliding gear transmissions. By using splined shafts, we obtain axial movement as well as positive drive.

Types:
There are several types of splines:

Parallel key spline:
where the sides of the equally spaced grooves are parallel in both directions, radial and axial.

Serrations:
where the sides of the equally spaced grooves form a "V". These are used on small-diameter shafts.

Helical splines:
where the equally spaced grooves form a helix about the shaft. The sides may be parallel or involute. This can either minimize
stress concentrations for a stationary joint under high load, or allow for rotary and linear motion between the parts.

Ball splines:
where the 'teeth' of the outer part are implemented with a ball bearing to allow for free linear motion even under high torque.

Manufacturing:

There are two types of splines, internal and external.
External splines may be broached, shaped (for example on a gear shaping machine), milled, hobbed, rolled, ground or extruded. There are fewer methods available for manufacturing internal splines due to accessibility restrictions.
Methods include those listed above with the exception of hobbing. Often, with internal splines, the splined portion of the part may not have a through-hole, which precludes use of a pull / push broach or extrusion-type method.

To prevent stress concentrations the ends of the splines are chamfered (as opposed to an abrupt vertical end). Such stress concentrations are a primary cause of failure in poorly designed splines

Advantages:
1-Can carry higher torque.
2-Lower stress on attachment (gear).
3-Better fit, less vibration (spline integral to shaft so no vibrating key)
May allow axial motion while reacting torque.

Disadvantage:
1-Cost.

Splines have the same
failure mechanisms as keys:
1) shear or 2) bearing.

Material selection:
A step in the process of designing any physical object. In the context of product design, the main goal of material selection is to minimize cost while meeting product performance goals. Systematic selection of the best material for a given application begins with properties and costs of candidate materials.
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