Technical Brief No. 32
All Technical Briefs are based on past projects at RH Lyon Division
Knitting
Needles
Fatigue in High Speed
Stress Analysis Machinery
Finite Elements Vibration dramatically reduces life expectancy
Abrupt accelerations result in failure tal and analytical procedures and can be illustrated
Many machines, such as those involved in the by the knitting needle example.
clothing industry, produce product at a rate propor-
tional to their speed. Sewing and knitting machines A experimental scale model of the knitting needle
for example have cam driven components that have was made. Because of the small size of the needle,
their positions defined by the it was "scaled up" by a factor of
cam shapes. The cams and How to………. 10:1, which means that the
followers may have been resonance frequencies in the
designed for displacement only, The fatigue analysis procedure model are reduced by the same
and the accelerations involved consists of the following steps: factor of 10. Small parts are
would have been acceptable at difficult to make vibration
the original design speed. As 1. Determine component inputs measurements on directly, but
the operating speed of the from a geometric analysis or a a scaled-up version of the part
machine is increased, the dynamic simulation of the can have exciters and sensors
accelerations increase accord- mechanism. directly attached so that reso-
ingly, causing greater forces 2. Carry out experimental tests to nance frequencies, mode
and resulting vibrations and the determine the high frequency shapes, and transfer functions
stresses they induce. modal dynamics of the part. can be determined. Such
3. Develop a structural scale measurements can be useful in
The accelerations can be very model. evaluating outputs from a par-
abrupt, having time constants 4. Create a finite element model of allel analytical study; for exam-
10 to 100 microseconds in the part and use the dynamical ple, a finite element calculation.
duration because of cam pro- inputs to predict stress varia-
files that look smooth to the The finite element calculation is
tions. valuable for predicting the vi-
eye, but may have discontinui-
5. Use a fatigue model (an S-N brational response of machine
ties on slope or curvature that
curve) to predict fatigue life. components due to cam or
will excite higher modes of
6. Modify the design (inputs or part other inputs. Displacements or
vibration of the knitting needle.
parameters) to reduce stresses accelerations can be compared
Such sharp impulses excite
and increase the predicted life. with measurements, but the
resonances in the cam follow-
ers and in the components finite element model will also
attached to them. The resonant predict stresses that determine
vibration of the industrial knitting needle is excited by fatigue life. Standard models of fatigue life for the
discontinuities of acceleration at its "butt" end, which material as they are affected by the processing
is directly connected to the knitting machine cam involved may not predict fatigue life precisely, but
groove. The “butt” is, in turn, excited by directions for design improvement can be derived
discontinuities of curvature in the cam profile. A from the calculations.
resonance in the needle can cause breakage of the
hook in the needle and a defect in the knitted The fatigue and breakage of small parts in high
product. speed machinery receives much attention in design
and production quality assurance efforts. We have
Experimental and analytical procedures used found this combination of analytical modeling,
The methods of analysis for the knitting machine accompanied by experimental studies, to be highly
case are analogous to those that would be used in effective in both determining the problem areas in
most other machines. They involve both experimen- the part and in recommending design changes.
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