JustinME
Mechanical
- Aug 20, 2013
- 27
Background Informatiom:
I have a problem that requires the input of impact force data into an FEA for structural design. I have a machine with two front wheels and one rear caster. The wheels and caster are suspected to have implications on strength requiresments of the machine frame; therefore, to minimize the total cost of frame, wheels, and caster I am investigating the effect of different wheel and caster designs on frame strength requirement. In other words, I can pay for a more expensive caster or wheel if it allows me to design a sufficiently less expensive frame. In this problem, I will only be dealing with the rear caster.
The machine CG (center of gravity) is between the front wheels and rear caster, and rather high so that ramp angles shift the machine weight between front wheels and rear caster.
The impact force of concern is the result of driving the machine off a three inch tall curb or ledge. To simulate this scenario, I placed the placed the front wheels of the machine on a lift table and I held the rear of the machine with a quick release trigger so that the rear caster had three inches below it and the "ground". Now what I refer to as the "ground" is actually a load cell that will measure force versus time. I also have an accelerometer mounted to the top of the caster mounting plate, so there is a load cell at the bottom of the caster and an accelerometer at the tope of the caster so elasticity of caster tread will cause some differences and/or lag between force and acceleration measurements.
The rear caster is attached to the machine via a stiff (not rigid) beam. The wheels are directly attached to the machine frame.
Test Conditions:
The machine began level with the ground and the rear of the machine was suddenly dropped three inches (rotational motion about the front wheel axis). A small DOE was run so that there was two different caster tread hardnesses tested and two different weights applied to the caster. The caster loading was measured at the end of each test run because the weight distribution had changed (ramp angle) and because the weights in the machine were launched slightly out of position (this represents real-world application, although not ideal for testing).
The low weight tested was 60 lb and the high weight tested was 90 lb.
The hard caster tread was Shore D 95 (phenolic material), and the soft caster tread was Shore D 15 (neoprene material).
Data:
Impact forces (load cell measurements) for both casters was approximately 700 lb for the low caster loading condition and approx 1050 lb for the high caster loading condition.
The data shows that the impact force measured is linearly correlated to the weight placed over the caster (at the end of the test, with the machine 3" out of level) with an r^2 value of 82.1%. Therefore, the wheel hardness had close to zero effect on measured impact force.
The soft tread caster had g-loads (acceleration) of 15 g's for both high and low caster loading conditions.
The hard tread caster had g-loads of 55 g's for both high and low caster loading conditions.
The data shows that tread hardness has a significant effect on measured acceleration during impact.
Problem Statement:
The force measured at impact is the same for the soft and hard caster tread, but the acceleration values are different by a factor of ~3.7. I do not know if I should use the measured g-loads (acceleration divided by gravity) or if I should use the force loads. I do not believe that the tread hardness has no effect on machine frame stress during impact, but my superior engineer (almost boss) does believe that the tread hardness should not effect impact forces the frame must support.
Questions:
1) What force should I use in my FEA in order to calculate stresses in the cantelevered beam that the caster mounts to?
2) Can anyone explain why the measured force is not effected by tread hardness?
3) My superior engineer said that the resonant frequency of the frame (i.e. system) that attaches the caster and wheels to the machine must have some resosnant frequency that forbids/prevents the tread hardness to eefect the impact force...which he knows because he used to do isolation designs for computer parts.
4) Is there a way for me to upload JPEG files of my Free Body Diagram, test results, etc. onto this forum?
Thanks in advance for your help!
Justin
I have a problem that requires the input of impact force data into an FEA for structural design. I have a machine with two front wheels and one rear caster. The wheels and caster are suspected to have implications on strength requiresments of the machine frame; therefore, to minimize the total cost of frame, wheels, and caster I am investigating the effect of different wheel and caster designs on frame strength requirement. In other words, I can pay for a more expensive caster or wheel if it allows me to design a sufficiently less expensive frame. In this problem, I will only be dealing with the rear caster.
The machine CG (center of gravity) is between the front wheels and rear caster, and rather high so that ramp angles shift the machine weight between front wheels and rear caster.
The impact force of concern is the result of driving the machine off a three inch tall curb or ledge. To simulate this scenario, I placed the placed the front wheels of the machine on a lift table and I held the rear of the machine with a quick release trigger so that the rear caster had three inches below it and the "ground". Now what I refer to as the "ground" is actually a load cell that will measure force versus time. I also have an accelerometer mounted to the top of the caster mounting plate, so there is a load cell at the bottom of the caster and an accelerometer at the tope of the caster so elasticity of caster tread will cause some differences and/or lag between force and acceleration measurements.
The rear caster is attached to the machine via a stiff (not rigid) beam. The wheels are directly attached to the machine frame.
Test Conditions:
The machine began level with the ground and the rear of the machine was suddenly dropped three inches (rotational motion about the front wheel axis). A small DOE was run so that there was two different caster tread hardnesses tested and two different weights applied to the caster. The caster loading was measured at the end of each test run because the weight distribution had changed (ramp angle) and because the weights in the machine were launched slightly out of position (this represents real-world application, although not ideal for testing).
The low weight tested was 60 lb and the high weight tested was 90 lb.
The hard caster tread was Shore D 95 (phenolic material), and the soft caster tread was Shore D 15 (neoprene material).
Data:
Impact forces (load cell measurements) for both casters was approximately 700 lb for the low caster loading condition and approx 1050 lb for the high caster loading condition.
The data shows that the impact force measured is linearly correlated to the weight placed over the caster (at the end of the test, with the machine 3" out of level) with an r^2 value of 82.1%. Therefore, the wheel hardness had close to zero effect on measured impact force.
The soft tread caster had g-loads (acceleration) of 15 g's for both high and low caster loading conditions.
The hard tread caster had g-loads of 55 g's for both high and low caster loading conditions.
The data shows that tread hardness has a significant effect on measured acceleration during impact.
Problem Statement:
The force measured at impact is the same for the soft and hard caster tread, but the acceleration values are different by a factor of ~3.7. I do not know if I should use the measured g-loads (acceleration divided by gravity) or if I should use the force loads. I do not believe that the tread hardness has no effect on machine frame stress during impact, but my superior engineer (almost boss) does believe that the tread hardness should not effect impact forces the frame must support.
Questions:
1) What force should I use in my FEA in order to calculate stresses in the cantelevered beam that the caster mounts to?
2) Can anyone explain why the measured force is not effected by tread hardness?
3) My superior engineer said that the resonant frequency of the frame (i.e. system) that attaches the caster and wheels to the machine must have some resosnant frequency that forbids/prevents the tread hardness to eefect the impact force...which he knows because he used to do isolation designs for computer parts.
4) Is there a way for me to upload JPEG files of my Free Body Diagram, test results, etc. onto this forum?
Thanks in advance for your help!
Justin
