GB2167523A

GB2167523A - Fatigue testing of springs

Info

Publication number: GB2167523A
Application number: GB08430013A
Authority: GB
Inventors: Alistair Peter Ward
Original assignee: Swansea University
Current assignee: Swansea University
Priority date: 1984-11-28
Filing date: 1984-11-28
Publication date: 1986-05-29
Also published as: GB8430013D0; GB2167523B

Abstract

A spring is fatigue tested by means of a vibrator armature which is vibrated at the natural frequency of the test spring, the spring being clamped under compression between clamping members 26, 27 rigidly coupled to the vibrator armature 26. The clamping members are preferably mounted on a rigid bolt 28 secured to, or integral with, the vibrator armature; the method is suitable for testing both open-end coil springs and whole coil springs. Apparatus for performing the method comprises a vibrator armature; a controller for controlling the frequency of vibration of the armature, the latter being rigidly coupled to the compression spring being tested; and means for providing a measure of the amplitude of vibration of the test spring. <IMAGE>

Description

SPECIFICATION Method of fatigue testing The present invention is concerned with fatigue testing of spring specimens.

Helical coil springs, such as automobile valve springs, are commercially shot peened in bulk. The shot peening is generally controlled by the inclusion of test specimens, the fatigue life of which specimens is periodically evaluated. It is known to use hardened, tempered steel strip test pieces, using the Almen test (as described in SAE standard J442), or test pieces in the form of a helical coil of spring wire (as described by A.M. Sanderson and R.G. Slingsby in Improved Method of Shot-Peening Control, Proceedings First International Conference on Shot-Peening, Paris, September 14-17, 1981).

In the method described by A.M. Sanderson and R.G. Slingsby, the test piece is vibrated with a low forcing signal power, tuned to the natural frequency of the test piece, thus enabling fatigue failure to be obtained after a relatively low number of stress cycles (typically 200,000 cycles at 310Hz).

The known apparatus for this method is illustrated in Figure 1 of the accompanying drawings, in which there is shown a test rig for testing a spring 1.

The spring 1 is clamped between knife edges 2, 3 acting on respective ends 4, 5 of the spring. Knife edge 2 is mounted on a clamping plate 6; the clamping plate is supported by a support helical spring 7 which is in turn supported by plate 8 secured to frame 9 by nuts 10, 11, 12, 13. The excitation force is applied to plate 6 via a light rod which passes through a hole in plate 8 and the balance spring 7.

Knife edge 3 is mounted on a fixed plate 16 which is also secured to frame 9 by nuts 17, 18, 19, 20.

The mounting arrangement illustrated is used for open-end coil springs; for mounting whole springs (that is, springs with closed-end coils), the knife edge mounting is not needed and the spring can be clamped directly between fixed plate 16 and clamping plate 6.

The spring 1 is vibrated by vibrator plate 8 under the control of control transducer 21 and an electronic controller (not shown) which allows the frequency of the applied excitation force to be continuously tuned to the resonant frequency of the spring 1. The amplitude of the movement in resonance can be measured using a travelling microscope 22, focussed on a "marker" (not shown) at the centre point of the spring 1.

The support helical spring 7 applies a static axial compression force to the test spring 1, thereby reducing relative movement between the test spring 1 and the knife edges 2, 3. This is effected to reduce dissipated energy losses sufficiently to allow the required amplitude of vibration to be obtained.

The vibrator plate 8 supplies only the dynamic component of the force applied to the test spring 1.

The apparatus described is complex and costly.

In addition, even through the support helical spring 7 is designed to have a different resonant frequency to that of the test spring 1, it has been found that there are undesirable "buckling" modes of vibration between the two springs in series, and that vibrations of the support helical spring 7 interfere with vibrations of the test spring 1.

According to the present invention there is provided a method of fatigue testing of a test spring, in which the spring is clamped under compression between clamping members rigidly coupled to the vibrator armature.

The testing of the spring under compression in this way enables static compression forces to be isolated from dynamic forces; vibration during the test can be restricted essentially to vibration of the test spring.

The test spring is typically a helical compression spring, and the clamping members are typically mounted on a rigid bolt secured to or integral with the vibrator armature.

The test spring may be an open-end coil spring, in which case it is generally clamped between knife edges mounted on the rigid bolt and secured respectively to the vibrator armature and to a nut.

Alternatively, the spring may be a whole coil spring, in which case, it is generally clamped between the vibrator armature and a nut, optionally with a washer between the nut and the spring and/ or between the armature and the spring.

Typical arrangements for clamping a spring in a method of fatigue testing according to the invention will now be described with reference to the accompanying drawings, in which: Figure 2 shows a clamping arrangement when used for a whole coil spring; and Figure 3 shows a clamping arrangement when used for an open-end coil spring.

Referring to Figure 2, there is shown a test spring 1 (in the form of a whole spring) held under compression between a vibrator armature 26 and a nut 27 by means of a rigid bolt 28 which passes along the axis of the spring 1. The nut 27 is screwed onto the screw-threaded end 29 of the bolt 28, and is separated from the spring 1 by a washer 30.

Referring to Figure 3 (in which like parts are identified by like reference numerals), there is shown a test spring 1 (in the form of an open-end coil) held between respective knife edges 31 and 32 mounted at opposite ends of a rigid bolt 28.

One end of the bolt 28 is mounted to a vibrator armature on which is mounted knife edges 31, while a nut 27 is screwed onto the screw-threaded end 29, the nut 27 acting on knife edge 32.

The clamping arrangements shown, with the rigid bolt passing through the axis of the spring, are those typically used for the excitation of "longitudinal" modes of vibration of the test spring.

Other appropriate arrangements are used for the excitation of other modes of vibration of a test spring.

The clamping arrangements eliminate the support used in the previous arrangement, avoiding problems associated with vibration of the support spring. The clamping arrangement is simpler, smaller, lighter and has lateral rigidity; and changes of geometry and compression are easy to implement. The degree of compression can, if desired, be changed during a test. The testing of the spring under compression enables a larger amplitude of vibration to be imparted than in the prior art method.

The method according to the invention is suitable for use in a method of quality control of shot peened helical coil springs. The method may be applied either during the manufacture of the springs or to finished springs.

The present invention has been described with reference to a method of fatigue testing; the invention further comprises apparatus for use in the method according to the invention, which apparatus comprises vibrator means having a vibrator armature and a vibrator-controlling means (such as a transducer and electronic controller), means for rigidly coupling a compression spring to the vibrator armature (such as means for clamping the spring to the armature); and means for providing a measure of the amplitude of vibration of the spring being tested.

Typical means for coupling the spring to the armature for use in the apparatus according to the invention are clamping arrangements as described above with reference to Figures 2 and 3 of the accompanying drawings.

Claims

1. A method of fatigue testing which comprises vibrating a test spring by means of a vibrator armature which is vibrated at the natural frequency of the test spring, in which the spring is clamped under compression between clamping members rigidly coupled to the vibrator armature.

2. A method according to claim 1, in which the clamping members are mounted on a rigid bolt secured to, or integral with, the vibrator armature.

3. A method according to claim 2, in which the test spring is an open-end coil spring clamped between knife edges mounted on the rigid bolt and secured respectively to the vibrator armature and to a nut secured to the bolt.

4. A method according to claim 2, in which the test spring is a whole coil spring clamped between the vibrator armature and a nut secured to the bolt.

5. A method according to any of claims 1 to 4, in which the spring has a longitudinal axis and the vibrator armature is vibrated transverse to said axis.

6. A method according to claim 1, substantialiy as described herein with reference to Figure 2 or Figure 3 of the accompanying drawings.

7. Apparatus for fatigue testing a spring, comprising: a. vibrator means having a vibrator armature and means for controlling the frequency of vibration of the vibrator armature; b. means for rigidly coupling a compression spring to the vibrator armature; and c. means for providing a measure of the amplitude of vibration of the test spring.

8. Apparatus according to claim 7, in which the means for controlling the vibrator armature comprises a vibration controlling transducer associated with an electronic controller therefor.

9. Apparatus according to claim 7 or 8, in which the coupling means comprises means for clamping the test spring to the vibrator armature.

10. Apparatus according to claim 9, in which the clamping means comprises clamping members mounted on a rigid bolt secured to, or integral with, the vibrator armature.

11. Apparatus according to any of claims 7 to 10, in which the means for providing a measure of the amplitude of vibration of the test spring comprises a travelling microscope focussable on a marker at a fixed point on the test spring.

12. Apparatus according to claim 7, substantially as described herein with reference to Figure 2 or Figure 3 of the accompanying drawings.