“Why is he going backwards all the time?” A technical look at the material breakdown.

You might have noticed operators are going backwards almost as often as they are going forward. This isn’t just because they like looking through a little camera screen. It also isn’t because they are pretending they are Mater from the movie, Cars. Although Mater was correct when he said, “Ain’t no need to watch where I’m going. Just need to know where I’ve been.”

There are two different primary functions in a mulching operation. First is the rough breakdown, that takes place going forward on most all front-mount mulchers, such as your typical forestry skid steer. Then there is the secondary breakdown, which occurs in the backward direction.

As the machine is moving forward the rotor is spinning simultaneously in a downward and forward direction. This motion drives the material down toward the ground, out under, and behind the machine. This is why it is generally a bad idea to stand directly behind a mulcher; you’re probably going to get something thrown at you.

During that downward and forward rotation, the material is not broken up very much, since it is just bashed against the ground and ripped backward. It is enough though that it allows the machine to keep on advancing. There are a few factors that can influence how much the material is broken down during that motion. The main factor is the type of material. Stringy material will tend to grab onto the teeth and hurl out under the machine without many breakdowns. The more brittle the material the more the tooth will break it into small chunks, instead of throwing it out under the machine.

So while the forward pass is necessary, it is not where the big value comes from; the back pass is where it all happens. During the back pass, the rotor is rotating upward, this lifts the material into the mulching chamber. The mulching chamber is the area from the back lower edge of the mulcher to the front upper edge. There are usually some type of fixed knife, solid bar, or “fingers” that stick out from the frame inside the mulching chamber that the rotor smashes the material into. Once the material enters the chamber, there is nowhere for it to go but continue through the maze and be thrown out on the other side.

Because the material is trapped it keeps going through a repeated series of getting hit by a tooth, then slamming into a fixed finger, then it gets slammed by a tooth again and hurled into another fixed finger. This rapid series of changes in velocity is what accomplishes the highest percentage of the breakdown of the material. The efficiency of an operator is ultimately based on how quickly they can get the material broken down into small particles.
Because of all this, you may even see an operator driving forward without engaging the head with the work at all, then they will drop it down and go backwards. This is because the time driving forward can be used to get the rotor back up to speed; thus storing energy. Then when it is dropped down and backpass, you get the highest degree of reduction possible with the energy available.

The backpass is also what gives that beautiful smooth look to a job. As the operator is moving backward, they can manipulate the door and or angle of the head to spray mulch from dense areas to areas where the mulch isn’t as heavy. This evens everything out, gets a better distribution of material for decay and breakdown, and covers the tracks.

There are a lot more factors that contribute to material breakdown, but the direction of the machine is probably the most noticeable to the outside observer and plays the biggest factor in turning big growing things into small particles of matter.

Below you can see an example of a forward and backpass with our 630 hp Prinoth Raptor on 20′ tall Christmas trees.