Shredding 101 - Shredder Basics

Theres a lot more to shredding than just cutters and shafts. Correct selection and application of a wide range of features will ensure a cost effective shredding solution.

Shredding 101 - Shredder Basics

Shredding 101 : Different materials require different shredding solutions.

Selection of the correct shredder for a particular application is vital for efficient size reduction, suitable throughput, and long term cost of ownership. The consequences of incorrect shredder selection include poor performance, increased maintenance, excessive downtime, and inevitably – increased costs.

There are many factors that need to be evaluated to ensure correct shredder selection including input material type, input material size, output size requirements, feed method and throughput requirements – all of which are essential elements of the correct shredder solution.

Input material type : does the material fracture/break when shredded or does it tear and rip.

Input material size : is the material bulky or flat, long or short ? Can the material be easily grabbed or does it need assistance to enter the cutting zone ?

Output size requirements : Are small uniform pieces required or are large/long pieces acceptable? What inputs does downstream equipment require ?

Feed Method : – Is the material to be batch fed, or will be a be a metered feed situation (such as a conveyor belt).

Throughput requirement : what volume/weight of material is required to be processed per hour.

Close Tolerance versus Open Table designs

Rotary shear shredders (such as our AZ Range) utilise the close tolerance between two sharp edges to slice through the material like a pair of scissors. Hooks on the cutters grab material and pull it towards the cutting zone to enable the cutter edges to slice through the material. These hooks also tear and fracture the material as it moves through the cutters however the primary reduction method is shearing. It is essential that these cutter edges maintain their sharpness which Brentwood achieves through the use of special alloy cutters which can be repaired and resharpened.

Open table designs (such as the range of Metso M&J Shredders) use aggressive knife designs to pull material onto a cutting table where it is torn and fractured into smaller pieces. These units do not rely on sharpened edges and can maintain their shredding efficiency even when the knives appear worn. These units are ideal for primary reduction where small, uniform output sizing is not essential.

Dual Shaft versus Quad Shaft

Dual Shaft Shredders are typically used where it is acceptable to have variation in the output size. These units, depending upon the input material, will tend to produce strips as wide as the cutters, and as long as the distance between the cutter hooks. Some longer pieces are also possible. When bulky items enter a dual shaft shredder’s cutting chamber, it is possible that the cutters may not grab the material resulting in “bridging”. This then requires intervention – such as a ram hopper – to force the material into the cutting zone.

Quad Shaft Shredders can be used where bulky items are in the input stream as the upper cutters act as a force feeder to the bottom set of cutters. When a screen is positioned under the cutters, the material is recirculated in the cutting chamber until it has been reduced to a size that will drop through the screen. While this reduces the throughput of the shredder by 30-40%, it does enable these shredders to produce a uniform sized output.

Cutters and Cutter Arrangement : Key variable that make a difference.

Cutter design and the arrangement of cutters on the shafts play a key role in determining shredding efficiency. The load on a shredder is primarily determined by the number of cutter hooks that are “engaged” at any particular point in time. The more hooks engaged, the higher the load. To manage the load in a shredder you need to manage the number of engaged hooks and this is done in a couple of ways.

The number of hooks on a cutter can be varied – single, dual, triple, six. The offset – or the degrees until the next adjacent hook, can also be varied. This enables the creation of spiralling of the hooks which is suitable for some applications – but not for all materials. Other applications require larger open gaps to enable large objects to enter the cutting zone. The correct use of equal speed and differential speed shafts is also essential to manage the load and create open areas. At Brentwood we design a cutter arrangement specifically to suit your materials and throughput requirements.

Brentwood’s cutters are manufactured from a nickel based alloy and case hardened to 60 Rockwell. This produces a cutter with excellent durability and superior shock tolerance so the cutter can withstand high impact loads which may occur when an unshreddable item enters the cutting chamber. Brentwood cutters are also repairable. We can hard face the cutting edges and regrind to specification several times over the life of a cutter. This greatly reduces the cost of ownership of a Brentwood shredder.

Cutting Chamber Length and Width : When size matters

The overall length of the cutting chamber also has an impact on the throughput of a shredder. Longer cutting chambers can increase production rates where the input material is flowable and can move in the cutting chamber. Too long a cutting chamber however can increase load beyond the shredders limit and place higher levels of stress on the shafts.

Cutting chamber width – and more particularly, the distance between the shaft centres plays a crucial role in determining if a material can be effectively grabbed by the cutters. If the shaft centres are too close for a particular sized input material, then bridging may occur or the shredder may have difficulty in grabbing the material.

At Brentwood, we offer customised cutting chamber lengths to enhance throughput on certain materials and to allow longer items – such as pallets and IBC’s to be effectively shredded.

Experience …. that only a manufacturer can offer

Having built heavy duty industrial shredders for over 40 years, Brentwood knows shredding. We have built shredders for almost every shredding application imaginable, and we continue to customise our shredders to suit new and challenging shredding applications. We work with our customers to trial materials at our factory and develop customised shredding, infeed, and discharge solutions. Discuss your shredding application with Brentwood today – if it’s not one of our standard designs, we will customise a shredder to your requirements.


Download our whitepaper “The Seven Secrets Of Shredder Success”  to learn the key factors that will make or break your shredding operation.



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