Selecting and maintaining a shredder

After purchasing a new shredder, whether to start a new waste and recycling business or expand an existing one, one of the biggest initial concerns is maintaining the machine to get the most out of the investment.

With a variety of shredder types and an even wider array of applications, no single set of rules applies to shredder maintenance. The most crucial factor in predicting the performance and longevity of a shredder is to select the right machine for the intended application. 

When shopping for a shredder, it helps to answer a few key questions:

• What material will be shredded?
• What size will the material be when fed into the shredder?
• What size will material need to be when it is discharged?
• How much material will be processed?

Two-shaft shredders

A two-shaft shredder is the most adaptable style of shredding technology. This type of shredder often is purchased because of its versatility, high throughput capability and low operating cost.

The versatility of two-shaft shredders comes from their low rotary speed and reversing capabilities, which reduce the risk of damage to the machine when it encounters nonshreddable materials. This makes them well-equipped to process contaminated material streams.

The design of the two-shaft technology lends itself to a low operating cost. Long cutting edges on the heavy-duty cutters spread the shearing action over a larger area, extending the machine’s maintenance interval. Lower rotary speeds allow the shredder to maintain power while consuming less energy.

Single-rotor shredders

A single-rotor shredder or grinder often is selected for shredding applications that require strict particle size control. Single- rotor shredders have a relatively lower upfront cost and, when properly employed, offer the most cost-effective option for uniform sizing of “clean” end materials. Material streams are considered “clean” when they are devoid of unexpected, nonshreddable contaminants.

Single-rotor shredders typically operate at speeds of 80 to 110 revolutions per minute (rpm) or higher—more than the 10 to 25 rpm of rotary shear shredders. Given the higher rpm and smaller cutting surface, cutters wear faster and require more frequent maintenance. Luckily, servicing a single-rotor shredder often is quicker and simpler when compared with its rotary shear counterparts. Cutters in a single-rotor machine can be rotated up to four times before needing replacement.

Four-shaft shredders

Four-shaft machines combine the versatility and robustness of a two-shaft shredder with the particle size control of a single- rotor shredder. Four-shaft shredders often are chosen for their ability to handle difficult materials while still delivering uniform particle size.

Four-shaft shredders also are rotary shear shredders, sharing many of the same advantages as the two-shaft machines.

While four-shaft shredders have a longer maintenance interval when compared with single-rotor machines, servicing them can be more expensive and time-consuming. 

Putting in the time

In short, single-rotor shredders require more frequent, albeit simpler and less expensive, service. Four-shaft shredders, meanwhile, require less frequent but more expensive and labor-intensive maintenance. Depending on the application, the relative operating cost can favor either of these styles.

When choosing which type of shredder is most appropriate, operators should consider whether the operation is set up for service. A company with an experienced service team or other equipment requiring service could find the shorter interval of the single-rotor shredder less of an issue. However, for companies that do not have the personnel to perform the maintenance, infrequent service could be more appealing and easier to manage.

How often a machine will require service depends on the application. Operational hours, throughput rate and the physical qualities of the material being shredded all play a role in dictating how often a machine will require service.

For example, a shredder processing mixed plastics could require a cutter change only once a year, while a tire processor with the same machine could need to exchange cutters every month. 

Maintenance schedule

Without a routine maintenance schedule in place, the best way to know when it’s time for maintenance is by monitoring the throughput rate. If productivity dips below an acceptable threshold, or if the shredder is reversing more than is tolerable, it’s a sign that it’s time for maintenance.  

Performing maintenance on a shredder means downtime. To optimize that downtime, plan to service or replace multiple shredder components at once. For instance, when changing cutters, check gears, fluids, bearings and seals ahead of time to see if they need replacing along with the cutters. 

In nearly all cases, it is cheaper and less time-consuming to employ a preventive maintenance plan than to wait for a critical failure. Operations managers at shredding facilities should allot time in the production schedule for shredders to be serviced and inspected.

Cutter health

When inspecting a shredder, it is important to pay special attention to the cutters. Cutters are critical components to maintain because they physically deliver the shearing or grinding action to the material. Keeping cutters sharp and in good working order allows the shredder to run as intended, maximizing production and reducing the load on the machine. 

An understanding of how shredding works reveals the importance of cutter health for the longevity of a machine.

In a rotary shear shredder, the hooks on the cutters grab and pull material through the cutting chamber, but the actual shredding happens from the shearing action of two cutting edges passing by each other.

“Shredders are scissors, not saws,” explains Auguie Henry, customer service manager for SSI Shredding Systems Inc., a shredder manufacturer based in Wilsonville, Oregon.

As the cutter discs wear, the once sharp cutting edges dull and begin to round.

“Sharp cutters are like hairdressers’ scissors; they cut easily,” Henry continues. “Dull cutters are like the blunt-tip scissors you find at an elementary school. They do more folding and stretching than cutting.”

Like elementary school scissors, worn cutters do more pulling than shearing. When this happens, production declines and the load on the shredder increases.

Rotary shear machines require not only sharp cutters but a tight cutter stack as well. A loose stack allows the cutters to shift during operation, causing accelerated and uneven wear.

In extreme cases, a loose cutter can even collide with an opposing cutter. When the stack is loose, dirt and other abrasive fine material work their way between the cutters and spacers, wearing away the face. Face wear makes cutters harder to rebuild, shortening their lives.

Single-rotor shredders create their shearing action when the tip of a square-shaped cutter passes through a fixed anvil. As the cutters and anvils wear, the gap between them grows, reducing the production of the machine. The increased gap also generates friction and heat, which can cause the shredder to jam and increase the stress on its components.

While preserving cutter health is important, other elements of the shredder should be monitored.

In the ongoing effort to reduce the load on the shredder, the machine’s cutting chamber and discharge chute should be inspected for blockages and cleared prior to starting the machine.

Shredders, like cars, use lubricant to reduce friction between gears in the drive train. Oil should be regularly inspected. Degraded oil or oil with a great deal of debris should be drained and replaced.

Old oil loses its lubricating properties, resulting in increased heat during operation. This heat can damage the gears. Additionally, contaminants such as metal shavings in the oil are abrasive and increase the wear on gears.

A multitude of factors, from application requirements and shredder specifications to personal preference, dictate the maintenance interval of a shredder.

Having clear production expectations can provide a threshold for triggering maintenance tasks.

Operators unsure about the best method for maintaining and optimizing shredder performance should contact experienced shredder professionals who can use their experience and expertise to inform your decision.

This article was submitted by SSI Shredding Systems Inc., based in Wilsonville, Oregon.