JABATAN KEJURUTERAAN MEKANIKAL

FAKULTI KEJURUTERAAN

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Lab 1 : Material lab

EXPERIMENT 1 : EXTENSION OF SPRING

1. INTRODUCTION

A helically coiled spring is an ingenious device which combines a very large range of force with anything from a little to a considerable elastic deflection. The fundamental principle involved is torsion of long rod wound round and round in the shape of a helix. Thus the number of variables involved must evidently be the rod diameter, the helix diameter, and the total length of rod. The performance of the spring is, however, measured by its stiffness, which is defined as the force required to produce a unit change in length of the helix.

Most springs are made of very high tensile strength steel wire which will remain elastic for a considerable twist. It also enables the ends of the springs to carry the applied axial force without distortion. Nevertheless for special conditions (for example corrosive atmosphere) one could use stainless steel, brass, copper or beryllium copper as all of these can be drawn into high strength wire.

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PROGRAM KEJURUTERAAN MEKANIKALFAKULTI INDUSTRI KEJURUTERAAN

LAB SHEET

2. APPARATUSThe apparatus consists of

Wall mounted channel fitted with a means of holding one end of a helical spring while a loading rod with a shaped hook is attached to the other end.

A load hanger is suspended from the rod. A clamp-on fiducial marker that can be fixed on the loading rod to indicated on a

50 mm scale the change in length of the spring when it is loaded. A range of helical steel wire springs. These springs have standard industrial ends,

being closed loops in a central plane.

3. OBJECTIVE To measure the stiffness of a spring and compare it with the theoretical value. To note how the stiffness is affected by the physical dimensions of the springs.

4. PROCEDURE1. To fit a tension spring hang it on the peg at the top of the channel. 2. Loosen the wing nut which clamps the fiducial marker on the loading rod and hook

the rod onto the bottom loop of the springs.3. Suspend the load hanger from the bottom of the loading rod. 4. Slide the fiducial marker into the slot by the 50 mm scale, zero the marker and

clamp it in that position.5. Load the spring by 5 N increments recording the change in length of the spring up

to the greatest readable deflection or the maximum load of 55 N. 6. Use a table as shown. Record the spring dimensions, and note how the spring

moves.

5. RESULTSOn one graph plot the deflection against load for each spring tested and draw the best fit straight lines through the points. The gradients of these lines are the inverse of the spring stiffness which should be calculated for comparison with theoretical values derived from.

Stiffness = W = d 4G∆ 8ND3

Where d = Wire diameterN = Number of turnsD =

=Mean diameter of spring coil(O/D – d)

G = Modulus of rigidity (77 kN/mm2 for spring steel)

6. OBSERVATIONS

Did the springs exhibit linear elastic deflections? Explain the fact that the tension springs did not extend for small initial loading. How well does the theory predict the spring stiffness?

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RESULTS

Spring data : Wire diameter mmSpring O/D mmSpring length mmNumber of free turns

Load(N)

Deflection (mm)Spring A Spring B Spring C

051015202530354045505560

Department of Mechanical Engineering, University Industri Selangor

Revised AUG:2005

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