# Calculate force loss

1. Jul 9, 2013

### thewoodviolin

I am new to the forum. I was calculating the force losses in a seat belt pretensioner. Here is the question.

A cylindrical plastic rod is pushed on one side at a force of 6367.2 N inside a metal tube. Find the energy loss (force loss or final force or output force) due to the compression of the rod.

Given
Initial pressure = 670 bar
Initial force= 6367.2 N
Original length of rod= 177 mm
Changing length of rod = 37.80 mm
Rod material = plastic
Youngs modulus of rod= 1.8 GPa
Inner dia of tube = 11 mm
Dia of rod = 9.6 mm

Last edited: Jul 9, 2013
2. Jul 9, 2013

3. Jul 9, 2013

### thewoodviolin

I want to find the final output force ór remaining force after compression happens (final output force should be less than the initial force).

4. Jul 9, 2013

### Simon Bridge

This is a case where you press on something that, in turn, presses on something else?
YOu need to model the system in a way that is amenable to analysis - like a free body diagram.

5. Jul 9, 2013

### thewoodviolin

Yes. You are right. When the above said force is used to push the rod, the rod in turn pushes through the blade of a gear (looks like turbine). I want to find the force in that area (just before it touces the blad of the turbine).

6. Jul 9, 2013

### pongo38

Are you sure about these figures: Original length of rod= 177 mm
Changing length of rod = 37.80 mm. That seems unlikely to me, unless I have misunderstood the situation. Does that mean that the rod shortens from 177 mm to 139.2 mm?

7. Jul 9, 2013

### Simon Bridge

I think you need to draw a diagram - what force? where? "area" of what?
Could it be that you are trying to work out an overall problem by sub-optimal means?
i.e. what do you need this force for?

8. Jul 10, 2013

### thewoodviolin

http://www.auto-medienportal.net/artikel/detail/21478/[/URL] [Broken]

The last picture from the above link explains it:

A force of 6367.2 N is applied on the green rod inside the tube.So the rod (plastic) runs through the turbine blade and stops after that. Now the remaining force is transferred through the turbine. You could see the rod is compressed in the last picture of the 4. A loss of energy (force) happens due to compression. I just want to find the remaining force (just before the force is transferred to turbine).

Initial force = 6367.2 N
Energy loss (due to compression) = ?
Remaining force = ?

Initial force - Energy loss = Remaining force

Last edited by a moderator: May 6, 2017
9. Jul 10, 2013

### thewoodviolin

@ pongo: Values doesn't matter. I just want to know the theoretical method to find the right answer.

10. Jul 10, 2013

### Simon Bridge

Dimensions don't match... you cannot subtract energy from a force and get a force.

The pic does not say what you want to do.

.. the first step is to draw a diagram - put arrows on it to show the forces you are interested in.

You need to understand how the pretensioner works - here's an example (not yours though)
http://auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/seatbelt4.htm
... and then you need to model it.
A compressible section would normally be modelled as a spring.

11. Jul 16, 2013

### thewoodviolin

I am still struggling to find the right answer. To put the question in a simple way;

Calculate the force required to compress (deform) the object? Just need to know the method to calculate.

12. Jul 17, 2013

### Simon Bridge

You've been told the method repeatedly.
Describe the process precisely, model as a spring, do a free body diagram.
Good luck.