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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.

Monish Shet

Monish Shete is CEO of Elscint Automation, a 30 year old firm manufacturing vibratory bowl feeders and similar small part handing products. He personally has more than 25 years experience with last 15 years being with Elscint. This blog will help customers and laymen understand the intricacies of vibratory bowl feeders. One can find information about the finer aspects of vibratory bowl feeders which cannot be found anywhere else. Case studies and information about various innovative ways of feeding and orienting of different parts are given in this blog.


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