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Research Papers

Evaluation of the Influence of a Planned Interference Fit on the Expected Fatigue Life of a Conjugate Cam Mechanism- A Case Study

[+] Author and Article Information
Pau Català

e-mail: pau.catala@upc.edu

Maria Antònia De los Santos

e-mail: tania.santos@upc.edu

Joaquim M. Veciana

e-mail: joaquim.maria.veciana@upc.edu

Salvador Cardona

e-mail: salvador.cardona@upc.edu
Department of Mechanical Engineering,
Universitat Politècnica de Catalunya,
Avda. Diagonal 647,
Barcelona, 08028, Spain

Contributed by the Mechanisms and Robotics Committee of ASME for publication in the JOURNAL OF MECHANICAL DESIGN. Manuscript received February 15, 2013; final manuscript received April 16, 2013; published online May 30, 2013. Assoc. Editor: Chintien Huang.

J. Mech. Des 135(8), 081002 (May 30, 2013) (8 pages) Paper No: MD-13-1082; doi: 10.1115/1.4024373 History: Received February 15, 2013; Revised April 16, 2013

Due to dynamic effects, clearances, manufacturing and assembly errors in form-closed cam mechanisms, the follower jump can also occur. For conjugate cam mechanisms a technique to avoid the follower jump without the use of a spring involves making the conjugate cam profiles bigger than the kinematical ones by adding an external offset. This strategy produces an interference fit between the conjugate cam profiles and the follower train. This paper presents an ordered procedure to study the influence that the planned interference fit has on the evaluation of the contact forces, the expected fatigue life of the rollers, contact pressures and the lubrication conditions. The study is based on a conjugate cam mechanism with translational roller followers used in a real automatic process for manufacturing muselets. A three-degree-of-freedom dynamic model is proposed and the Hertzian theory for general profiles is used to model the nonlinear contact stiffness between the cams and the crowned rollers. The dynamic model predicts that it is difficult to obtain conjugate cam mechanisms with an infinite expected fatigue life of the rollers just by considering typical achievable manufacturing errors or clearances, and as happens in reality, a set-up process is highly recommended. The procedure is also tested with measured manufacturing errors on a coordinate measure machine—CMM—and with measured radial internal clearances for the rollers measured by an experimental apparatus. Also, to evaluate lubrication conditions, surface finishing measurements have been taken of the cams and the rollers with a surface profiler.

Copyright © 2013 by ASME
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Figures

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Fig. 1

(a) Linear displacement functions and (b) kinematical conjugate cam profiles

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Fig. 2

(a) Real conjugate cam mechanism; cam 1 and the upper roller are behind the train follower and (b) dynamic model

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Fig. 3

Rotation speed of the upper roller

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Fig. 4

Rotation speeds of the rollers for a complete rotation of the cams

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Fig. 5

Equivalent dynamic load

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Fig. 6

Expected fatigue life of the rollers

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Fig. 7

Contact pressure for different values of contact force

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Fig. 8

Film thickness variation for different ranges of dynamic equivalent loads

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Fig. 9

Normal distribution for the interference fit for the conjugate cam system

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Fig. 10

Monte Carlo simulation with a planned external offset of 37 μm

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Fig. 11

Measuring the cam profile 2 using a CMM

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Fig. 12

Manufacturing errors along the profile for both cams

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Fig. 13

Experimental apparatus for determining the radial roller clearances

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Fig. 14

Interference fit obtained with measured data

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Fig. 16

Contact pressures

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