At the Garden Shredder Review we are frequently asked – “What is the difference between rapid, quiet and turbine cut shredders”?
It is something I thought we had covered on the web site but it seems not well enough. So, in this article, we are going to review the definition of each type of shredder, how they work, and finally, highlight the differences.
As usual I am going to discuss the Bosch range of shredders. All the other manufacturers seem to employ very similar technologies do the answers should transfer very well.
Definition of a Rapid Garden Shredder
The most popular shredders are all Rapid machines. They are also often known as impact or rotary shredders.
All Rapid shredders use a high speed electric motor driving a single, high speed spinning blade.
The cutting mechanism is very simple. There is a single, rapidly rotating blade. . The blade will easily chop through smaller and softer clippings. Bigger, harder branches are chopped and chipped by the blades.
The effectiveness of the shredder relies on maintaining the speed of rotation of the blade – faster rotating blades hit waste with the highest possible force – and keeping the blade sharp.
The sharper the blade, the more effective the chopping impact. High speed sharp blades tend not to get stuck in any woody material so the rotational spped is each to keep high.
Rapid Shredder Noise Levels
High speed motors and spinning blades equates to a pretty high noise level. The Bosch AXT 2200 Rapid is rated at 107dB. This is around the sound level at a middle of the road rock concert. It is well above the danger level.
Problems with Rapid Shredders
Rapid shredders work great for relatively small volumes of garden waste and clippings. Problems start when we move up to heavier, woody source materials.
Unlike their heavier counterparts, rapid shredders have no self feed mechanism. For softer, green waste you need to push it through the hopper with the supplied plunger (or in my case, a good stout stick I have kept back just for this purpose). Smaller, woody clippings can be put in the hopper and will gravity feed through the machine.
The feed mechanims works well but can be slow. The throughput of rapid shredders (kg/hr) is much less than half that of a quiet or crusher shredder.
The real problem comes with bigger, woody clippings. Rapid shredders reduce woody clippings through the high speed impact from the rotating blades. Where the blade cannot cut through the cutting in one chop they will usually chip away until they get through. Again, this works well but it is noisy and slow. Most operators tend to ‘help’ the cutting through the machine. This extra help tends to attempt to present too much of the cutting to the blade in one go causing the machine to stall and jam.
OK – So What About Quiet Garden Shredders?
First of all – there is no actual, agreed definition of a Quiet Shredder. It will mean different things to differing manufacturers. .
To get noise levels down the ‘Quiet’ range of shredders uses a completely different shredding mechanism. The high speed blade is replaced with a much slower motor and gear train. The high speed blade is replaced with a relatively slow speed cog wheel.
Teeth on The Cog Crush and Cut Against The Side Plate
As waste enters the shredder is is fed onto the teeth of the cog wheel. As the cog rotates it pushes the waste against a striking plate on the side of the shredder. As the wheel continues to rotate the waste is simultaneously crushed and, as the teeth dig in, cut into small pieces.
The gear train is able to generate a lot of torque which helps keep the cog turning pretty much whatever you put into the shredder. The torque and the action of the shredder creates a lot of force which is quite capable of cutting and crushing quite large branches up to about 40mm – 45mm in diameter.
The action of the teeth grabbing woody waste means the Bosch Quiet Shredders are pretty effective at self feeding. Once you feed a branch into the machine the shredder will pull it through.
Of course, the machine has to be quite beefy to withstand the forces generated in slowly crushing and cutting branches. These machines are built very ruggedly. They typically weigh 2-3 times more than an impact shredder.
So, Are Quiet Shredders Actually Quieter?
The rated noise level of the Bosch AXT 25D is 88dB. That’s 19dB less than the Bosch AXT 2200 Rapid rating.
What does that mean?
This section is a bit technical. Feel free to skip it.
Decibels (dB) measure sound intensity – not volume or loudness. To complicate things further, the Decibel scale is logarithmic. A change of 3dB represents a doubling (halving) of sound level. To get a perceived doubling or halving of volume requires a change of around 6dB.
The rapid shredder produces 19dB more sound than its quieter brother. That makes it seem to be 8 times louder than a quiet shredder. You absolutely should be wearing hearing protection whenever you use a rapid shredder
Put another way – the quiet shredders generate 1/80 the sound produced by their more raucous cousins. That is a pretty phenomenal change in noise output.
Problems with the Quiet Shredder Range
Whereas the high speed cutting action of the rapid shredders is well suited to both leafy and light woody waste the quiet shredders only really deal with woody waste. When fed too much leafy waste a couple of things happen:
- a lot of the leafy waste simply drops through
- the leafy waste gets compacted into the valleys between the teeth on the cog wheel. Eventually the cog wheel becomes clogged and stops working.
Clearing the teeth once they have become clogged is a time consuming, tedious process.
Turbine Cut Garden Shredders – All Problems Solved?
Bosch’s Turbine Cut Shredders stay quiet by keeping the rotation speed slow. However, they deploy a completely different mechanism to deal equally well with woody and leafy waste.
To me, the Turbine Cut mechanism is very like that of a cylinder lawn mower. There are 8 blades mounted on a rotating, conic cylinder.
As with the teeth in the quiet shredders, the blades on the cylinder grab cuttings and clippings and forces them against a fixed side plate.
Larger branches are grabbed by the blades and forced against the side plate. Once the waste is snagged against the side plate the torgue on the blades is sufficient to cut even larger branches – up to 45mm. As with quiet shredders, the blades pull cuttings through the machine as they are forced against the first side plate. The turbine cutter shredder has a very strong self-feed mechanism.
Smaller, softer, leafy waste may drop between the blades. In doing so any stems will still be pulled over and cut against the left hand side plate (as we look at the diagram) and is chopped up by the blades. Anything that manages to reach the lower blades is caught and swept up against a smaller plate on the bottom right (again as we look at the diagram). Smaller woody waste is managed in exactly the same way.
The noise level is pretty low too – 82dB. That is something like the sound of city traffic heard from inside a car.
The Turbine Cut shredders are just about the perfect electric shredder. Very quiet, high throughput and can deal with just about any sort of shredding waste.
The downside is the cost. At over £400 these cost more than buying a Rapid Shredder for your general garden waste and a Quiet shredder to deal with branches.
As you know, we love the Bosch AXT 25TC. It is our shredder of the year 2019.