Mechanical Engineering 101 project - help?

[jon-mullen]

Hey guys,

I am working on an ME 101 project and could use some help. The basic aim of the project is to use a ShopVac (wet/dry vacuum cleaner) set on blow to pump water. My group has chosen to use a blower wheel (like from a cage blower fan) as our input and a reciprocating pump as our output. We would have a pinion on the same shaft as the blower wheel hooked up to a gear on the shaft with the crank.

Using weights, I found that the pump needs 2 lb. (approx 10 N) of force to make it move. I am trying to find the power/torque/force at the blower wheel so I can pick a gear ratio. I do not have fancy tools.

I think that I can measure this by connecting a rubber band with known spring constant to the shaft with the blower wheel and applying the ShopVac. The rubber band would be connect so without the ShopVac applied the rubber band would be just before the point of deflecting. When the power is on, the rubber band will deflect to balance the force.

I think that if I then marked the spot on the rubber band touching the shaft, and compared that length to the length just before it deflected, that difference multiplied by the spring constant would give me the force, right? Then multiplying that by the radius of the shaft would give me the torque?

Any help would be greatly appreciated. I do not have the resources or budget to try multiple gear ratios, and our final grade is based in part on how quickly our machine pumps a certain amount of water, so I am trying to get just enough force at the crank to move it without sacrificing any angular velocity.

Thanks in advance!!

[xpensive]

An easier way might be to measure the electrical motor's power and divide that with the Rpm, which should give you a fairly accurate estimation.

Torque (Nm) = Power (W) / Angular velocity (rad/s)

Don't forget the efficiency ration of the electrical motor.

[jon-mullen]

xpens, thanks for writing me back. The ShopVac has stats:
Peak HP: 5.5 HP
Sealed Pressure: 59" (~2.0 psi)
Air flow: 195 cfm

I had made an estimate earlier based on the pressure. The blower wheel always has 1/2" sq. exposed to the wind, so I figured the force there would be 1 lb. (2 lb/sq. in. * 1/2 sq. in. = 1 lb). But that method didn't seem as direct as using a spring to measure the force.

Last night I tried it with the rubberband. I did it with the rubberband attached to the blower wheel instead of the shaft. There was measurable deflection and today I'm going to determine the spring constant with some known weights. I'll keep you posted.