Forces acting on a screw

@zaveri-5TD6Sk • Oct 22, 2024

Oct 22, 2024

1.1K

Consider this situation:

a blade of a fan has to be attached to its hub, using two screws. now how do you go about designing them.

the data available to us is:

weight of the blade.
diameter of fan hub.
maximum speed of rotation in rpm.

Like 0 Replies 6

Replies

Welcome, guest

Join CrazyEngineers to reply, ask questions, and participate in conversations.

CrazyEngineers powered by Jatra Community Platform

zaveri

@zaveri-5TD6Sk • Sep 15, 2012

One approach is, to first determine the forces that will act on the screws and thus the stresses that they will induce.

now whenever the the fan starts from rest, the weight of the blade, due to inertia, exerts a shear force on the screw. thus this induces shear stress in it.

now the centrifugal force, due to rotation of fan, too may be inducing stresses in the bolt. thus the bolt material can be chosen so as to withstand the maximum of the two stresses.

is this right ? #-Link-Snipped-# #-Link-Snipped-#, what do you have to say to this.
Ramani Aswath

@ramani-VR4O43 • Sep 15, 2012

You are right about the forces you indicated.
The blades are usually asymmetric. This leads to a difference in the force acting on the two fixtures (The blades are usually riveted). The drag caused by the blade moving the air creates additional shear. The downward thrust of the air flow tries to push up the blade. This leads to a tensile force. This should be considered at the highest speed of the fan. When the fan is stationary the weight of the blade acts downward in ceiling fans. This also creates a small tension.
Jeffrey Arulraj

@jeffrey-xA7lUP • Sep 15, 2012

each action has opposite response this i think provides the balancing effect to the centripetal and other forces acting in the screws of the fan
Ramani Aswath

@ramani-VR4O43 • Sep 15, 2012

jeffrey samuel
each action has opposite response this i think provides the balancing effect to the centripetal and other forces acting in the screws of the fan
The opposite action in this case are the shear and tensile forces described above. The fixture should take these forces with an adequate factor of safety.
Jeffrey Arulraj

@jeffrey-xA7lUP • Sep 15, 2012

thank you sir
CE Designer

@ce-designer-BWq0yl • Sep 17, 2012

#-Link-Snipped-# both you and ramani covered the loading criteria pretty well. Free body diagrams always help me.