Thursday, March 3, 2011

CrazyEngineers Forum - Mechanical & Civil Engineering

CrazyEngineers Forum - Mechanical & Civil Engineering

Strain Gage for Measuring Load on Shaft

Posted: 03 Mar 2011 01:24 PM PST

Hello everybody,

I'm new here, have visited on several occasions and lurked in an effort to learn - but this is my first time posting.

I'm an engineering student working on a project and am working with strain gages for the first time. The project consists of a rotating aluminum shaft (3 mm in diameter, 10 mm in length), with a motor on one end and a variable load at the other (load varies between 0 to 8kg). I'd like to mount a strain gage onto this shaft to be able to measure the load.

The shaft does not rotate continuously, only back and forth within a range of approximately 120 degrees. My thought is to model this problem as a bending load at the end of a shaft and determine the deflection in the aluminum shaft. Doing this, I might be able to spec out a strain gage. My problem is I'm not sure how to use these deflection / stress calculations to determine an appropriate strain gage - I've been looking through the Omega and Vishay catologues. I'm also not sure if it's even reasonable to expect to find a strain gage that is small enough to fit on a 3mm shaft, that has a resolution capable of measuring such small deflections.

Would anyone familiar with Strain Gages be able to provide a little insight / help for a lost student? :) Thanks!

Thrust Runner Design

Posted: 03 Mar 2011 01:19 PM PST

The results of sectional-3D FEA on a Thrust Runner of the Thrust Bearing in a Pump shows that the Runner "bows" under the thrust load. The axial displacement of the periphery of the Runner is ~60 µm. The thickness of fluid-film between the Thrust Runner and Tilting Pads is estimated to be ~10 µm.

Radial interference of 5 mils/inch diameter is modeled between the Shaft and the Runner. The applied loads are: (i) Interference, (ii) Centrifugal, and (iii) Axial Thrust (mapped from 1 Tilting Pad on to the Runner Surface).

I am concerned that the excessive deformation on the periphery of the Runner will cause the lubricant film to be unsustainable, and drastically reduce the load-carrying capacity.

My question is, how much displacement is allowable on the Runner?

Hooke's Law Explanation & Practical Applications

Posted: 03 Mar 2011 05:03 AM PST

Definition: Within the elastic limit of a solid material, the deformation, produced by a force of any kind is proportional to the force.

Has anyone wondered why the definition has 'within the elastic limits of solid material' in the definition? If we go by the equation, it would be a linear equation - any reason why?

I'm starting this thread as a pointer to all the mechanical engineering concepts that can be discussed via this section. I look forward to responses from all mechanical engineers :)
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