Shredders Design

June 7, 2012
 

Vincent offers two series of shredders, the traditional Series VS (Vincent Shredder) and the newer Series VCS (Vincent Comb Shredder).

Not many years ago we found that the shredders work better if the blades are 180 degrees apart instead of 90 degrees.  We made them 90 degrees for decades, thinking they would be better balanced.  However, it turns out that balance is not affected.  What does happen is that capacity goes up because there is a longer interval between blades swinging past the inlet, so material falls deeper into the shredder before it is hit by an oncoming blade. 

Today we make all our shredders with blades 180 degrees apart.  Discussion with Corenco, a shredder specialist company, gave us the confidence to make this change.

On the larger 18" models we use one expansion pillow block bearing and one non-expansion pillow block bearing to support the rotor.  The non-expansion bearing goes in the inboard position, closest to the coupling.    The rotor does not expand from heat, but it can move from shock and distortion.  So we lock one end of the rotor and let the other float.

The 18" shredders use Dodge ParaFlex couplings between the motor and rotor.  This keeps shock from the rotor from being transmitted to the motor.  The ParaFlex is like the tire of a small car.  One rim is clamped to the rotor shaft and the other rim is clamped to the motor shaft.  These are rather expensive, but they are the best coupling for the application.

The inlet hopper of the shredders used to be the width of the rotor by one half of the diameter of the rotor.  That is, it was a rectangle located over one half of the rotor.  The idea was that the rotor would turn so that the material, which all fell on one side of the rotor, would be thrown down toward the discharge of the shredder.

Ironically, we had some models where we ran the rotor in reverse, so that the tendency was to throw most of the material up and out of the shredder.  This arrangement suited certain applications.

About ten years ago we found out that it works fine if there is a square inlet to the shredder, measuring the width of the rotor by the diameter of the rotor.  This design does tend to throw material up and out of the shredder, so we always expect that there will be something to stop this material from flying out.  This "something" would be the cover over a screw conveyor which carries material to the shredder, or a dog-leg chute over the inlet to the shredder.

This square inlet is less expensive to manufacture.  Also, it eliminates the job of removing a worn rotor from the shredder and turning it 180 degrees around.  People used to do this in order to use the other (non-worn) edges of the rotor hammers.  Now, when the teeth are worn on one side, it is necessary only to switch two of the electrical leads going to the motor, reversing the direction of rotation.  A Florida engineer, David Tegreene, was the first to come up with this.

It was common practice to put metal traps on the side of the shredder housing.  These would fill up with the material being shredded, which was soft material.  When a piece of tramp metal or a rock came along, the idea was that it would be thrown into the trap or pocket on the side of the housing.  This tramp material would imbed itself there, to be removed periodically.  Today we generally do not bother putting these traps on our shredders.

Note that with the original rectangular inlet, there was need for a trap on only one side.  With the square inlet, two traps were needed since the machine could be operated with the shredder going in either direction of rotation.

The 18" diameter shredders with direct coupled motors run at 1800 or 1500 rpm.  The smaller 12" and 8" shredders run at 3600 or 3000 rpm.  We have also built a lot of shredders with belt drives between the motor and the rotor.  Today we use the direct coupled motor since it is less expensive to build.  An advantage of the belt drive was that it provided a safety factor in the event that the rotor jammed while the shredder was in operation.

Most VCS comb shredders operate without the comb being used.  This is because the main trouble with the comb shredder occurs when a piece of heavy tramp metal enters the machine.  The rotor will jam and stop so suddenly that inertia of the rotor will break the cast iron housings of the pillow block support bearings.  We have found that the VCS shredders work well, in many applications, without the comb.  They do an adequate job of breaking up material so that it will effectively pass into a screw press.

There are some shredders or hammer mills built where the hammers are held by a single pin between two plates.  These hammers can pivot when they hit the material being shredded.  Vincent never did this.  All of our shredders have had hammers or blades that are rigidly mounted to the shaft.  This rigidity was achieved by either using two pins between parallel plates to hold the blades or by welding or keying the blades to a shaft.  Today we almost always use keyways to hold the blades to the shaft. 

Where the blades were welded to the shaft, the construction consists of a tube which is slipped over the shaft.  The blades are welded to this tube.  The tube shrinks from the welding operation, firmly gripping the shaft.  In this manner stress risers, from the welding, are avoided in the shaft.

Vincent makes blades or hammers out of a 410 series stainless.  This alloy is used because it can be hardened by heat treating.

Issue 246