Tub grinders were first introduced for wood and green waste recycling applications in the early 80’s. They were originally designed to recycle logging slash and lumber mill waste by reducing the waste wood into boiler fuel. Soon after development many landfills employed tub grinders for wood and greenwaste reduction purposes. Also, many composting operations arose to make use of the new source of mulch for a carbon source in their operations. Finally, the landscape mulch began to develop and replace much of the bark and pine needles that were common place previously.
There are a variety of sizes and horsepower, designed to handle land-clearing debris, bark, brush, logs, stumps, pallets, wood construction and demolition (“C&D”) material, leaves, as well as a large variety of other wood and greenwaste applications.Many tub grinder operators produce salable product such as mulch, colored mulch, boiler fuel and mulch for composting. Other operators use their grinder to reduce the volume of greenwaste material to minimize the consumption of expensive landfill air space, or reduce trucking costs and save on fuel and dump or tipping fees.
Tub grinders are aptly named for the distinctive infeed hopper which is simply a large round tub, normally 10’-14’ in diameter and roughly 6' deep. Production rates and capability of the tub grinder vary primarily with the weight, horsepower and size of the tub grinder. Smaller grinders may produce 20 yards of finished product an hour while larger grinders are capable of processing several hundred yards of finished product an hour. Production rates for each grinder will vary greatly depending on several factors. The primary determinants of production include: the type of material processed, the size of the finished product desired, support equipment available, and contaminants in the raw material..
Most tub grinders are powered by diesel engines, although many manufacturers also offer electric options (usually stationary). Caterpillar is the most common brand of engine found in larger tub grinders, although Cummins and John Deere are used frequently as well. Horsepower ranges from the mid 200’s to as high as 1200 in the largest machines.
At the base of the tub is the cylindrical shaped hammermill which is the primary grinding mechanism in the machine. The hammermill typically rotates around 1800 rpm. Some machines employ belt drives or gear boxes to effect the mill speed. Most tub grinders utilize a "Pin & Plate" style hammermill that consists of a prmary drive shaft, mill plates, rods and hammers.
Beneath the hammermill are rolled steel screen plates (usually around 1" thick for wood waste applications) with ½” to 6” holes cut in them which permit the material to pass through once the desired product size is achieved. A conveyor belt or augers located below the screens transports the finished product away from the mill to the discharge conveyor which carries the material away from the machine. Tub grinders are commonly mounted onto a rubber tire mounted frame with a fifth wheel-hitch. In some cases pintle hitches are offered for smaller machines and some tub grinders are available on tracks.
Tub grinders are typically loaded from the top with a front-end loader, grapple or excavator. Many units are also equipped with a self loading grapple and cab.
Typically the raw material does not need to be arranged in any particular fashion for proper feed or operation. Tub grinders rely primarily on gravity to feed the material into the hammermill at the bottom of the tub. Additionally the tub walls rotate around the hammermill and tub floor to pull material around to the hammermill. This tub rotation provides the operator with a way to control the speed at which the machine feeds raw material in the tub to the hammermill.
This is intended to be a brief introduction to tub grinders. For more information about finding the right tub grinder for you operation see the articles under the "Find the Right Machine for the Job" category.
If you are interested in the history of grinder manufacturers there is a nice article over at BioCycle. Tub Grinder Manufacturer History.