20 December, 2009
Leaf Springs for Vibrators
Flat leaf springs used in vibratory feeders are subject to very high bending forces in a single plane. They must withstand high fatigue conditions as they are expected to deflect many millions of times during their lifetime. Springs of vibratory feeders deflect more than five million times a day in certain cases. Due to this, they need to be made of the correct material as should have high fatigue strength. The speed and performance of a vibratory feeder in many cases depends upon its springs. They need to have the correct length and width as well as thickness. In case the length is more, the chances of breakage increase. In case the width is more, the performance is affected, while in case of lesser width, the chances of breakage increase manifold. Similarly, in case of very less thickness, the springs break a lot while higher thickness will result in lesser vibrations and lesser speed for the vibratory feeders. Usually, leaf springs of vibrators are made of either spring steel or fibre or composite material. There are various advantages and disadvantages of using either of these. The properties of Spring Steel and Composite Springs are given in the following table-
Property Spring Steel Composite / Fibre Units
Flexural Strength 932 760 MPa
Flexural Modulus 28 22 GPa
Tensile Strength 800 480 MPa
Tensile Modulus 33 23 GPa
Compresive Strength at 0 degree 724 690 MPa
Compresive Strength at 90 degrees 315 690 MPa
Maximum stress for infinite fatigue life 138 100 MPa
Thermal Conductivity 0.34 0.34 W/M K
In this table, it is assumed that the correct type of processes have been undertaken on spring steel. As can be seen, spring steel springs are more effective in all respects vis a vis composite or fibre springs. Price of spring steel springs is almost 20% of that of composite springs. Additionally, one needs double the total thickness of composite springs as compared to spring steel springs, further increasing the cost.
If the spring steel springs are so effective, then why do some manufacturers still use composite springs? This is a good question. The answer is that in case of half wave bowl feeders, spring steel springs are of lesser thickness and that too only a few are required. This increases the stress on the springs and they break quite often (even once a day!). Hence, in such cases, composite springs are used in order to increase the thickness and reduce breakage. Secondly, in case of full wave bowl feeders, the springs are in a bunch and the stress on them is very less. This reduces their chances of breakage. Hence, to sum up, springs steel springs are the best for vibratory feeders.