Loads using pulleys in a deflection test apparatus.

[dakirk08]

I am desiging a simple deflection test apparatus for my company.

In the system I am using a few pulleys, a steel cable, a force gage, and a ratchet strap.
I want to use the ratchet strap as opposed to dead weights so I can apply varying amounts of force to the test member.

My question is...If I apply a force of 10 lbs to the ratchet strap so that the force gage reads 10 lbs of force; is that comparable to hanging 10 lbs of dead weight from the test member. Or, will the use of pulleys change the amount of force. My assumption is that the pulleys will only change the direction of the force, not alter it.

Any input would be greatly appreciated. Please see attached diagram.

View attachment Deflection Test Apparatus.pdf

 

[AlephZero]

Assuming the tension in the whole cable is uniform and the two parts of the cable up to the load are parallel, you would have a 20lb force on the test speciimen for a 10lb tension in the cable, because the cable is doubled. Look at the section on levers and pulley systems in a mechanics textbook if you don't see why.

For a practical setup, I would suggest yuu run the cable over another pulley hung from the nylon strap to reduce the friction in the system. In your drawing, the friction at the clip, together with the sharp bend in the cable, will probably make the tension in the cable on each sides of the clip different, and then you don't know what total load is being applied with any accuracy. You would probably find if you plotted the test results as you increased the tension and then decreased it again, the two curves would be significantly different (and of course they should be the same).

 

[dakirk08]

Thank you very much. I thought my logic may be a little off. The textbook that I have "Physics" by Paul E. Tippens just touches on pulleys, and I kept running in circles in my logic. The reason I was using the spring clip instead of a pulley was simply because that's what home depot had. They were out of the other pulleys, but I will go back and see if i can find one.