Rent Smart: Smooth operators

Currently, the 3-ton smooth tandem drum vibratory compactor with a drum width of 47 inches is the most sought-after rental machine in this category because of its compact, yet versatile size, according to rental companies. “It offers enough maneuverability, productivity and basic capabilities to satisfy most needs,” says Bill Locklin, vice president, southwest region, United Rentals.

Other popular choices include larger compactors with drum sizes that range from 67 to 84 inches. “These size classes have the weight and drum widths that fit many of today’s roadways and paving widths,” says Terry Sharp, manager of solutions and services for Caterpillar’s global paving division.

Before renting a compactor, figure out the necessary drum width for your job. Sharp says choosing the wrong drum width is the most common mistake renters make. A too-large drum width prevents the compactor from making the correct rolling pattern and leads to uneven mat densities.

Shake down
To meet target densities, you need to select a rental machine with a vibratory system that hits hard enough to manipulate materials. Vibratory characteristics, such as amplitude – or the measure of total peak to peak vertical movement per drum cycle, according to Caterpillar’s Soil and Asphalt Compaction Manual – and frequency – or the number of times the drum strikes the surface – will determine compaction efforts. “Impacts of 10 to 14 per foot are desired for most paving applications,” Sharp claims.

Several compactors come equipped with amplitude settings that adjust to asphalt and soil conditions. For asphalt compaction purposes, units with a vibrating front drum, rear drum or both work best. Versatile vibratory systems offer low amplitudes for thin lifts and tender mix designs, while high amplitudes are more commonly used for harsh mix designs and thick lifts, Sharp explains.

Newer developments include units that use “intelligent compaction” methods where a computer analyzes surface conditions and then adjusts the amplitude and exciter speed to match those conditions, Locklin says.

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Keep on rollin’
“For a paving project to move forward as efficiently as possible, compaction speed must match paving speed,” Locklin notes. But renters beware: You must match the roller’s compaction settings to the particular surface it’s compacting. According to Cat’s compaction manual, when it comes to asphalt, knowing the temperature of the mat passing under the paver’s screed is important. If the mat cools below optimum compaction temperatures, the roller will have little effect on meeting required density targets, Sharp says.

The grade of asphalt, aggregate type and the viscosity of asphalt cement all affect the optimum rolling temperature. Cat’s manual says most often compaction should begin at a high temperature, since density can be obtained with fewer passes than a low temperature may require.

To be safe, watch the mat temperature and it will emphasize how closely the paver and the compactor should operate. Rapidly cooling mats mean a short amount of time to achieve required density.

Renting a high-frequency vibratory roller (one with a frequency of 4,000 vibrations per minute or higher) allows the roller to increase its speed without compromising surface smoothness. “This lets the roller keep up with faster, more productive pavers,” Locklin says.

Perimeter frame vs. vertical leg
Other options with compactors include perimeter frame models, which use a rigid-box design with a suspended drum, or vertical leg models, which feature an open frame. “Perimeter frame compactors are the mainstay in North America,” Locklin says.

The drum supports the frame, thus supporting the operator’s platform, engine and other components. But perimeter frame machines also tend to have a lower profile that can limit drum edge visibility. “This makes control near curbs and obstacles difficult,” Sharp says.

As for advantages, visibility to the drum surfaces can be better with this type of compactor, and the lower profile puts more non-vibratory static weight on the drum, which allows it to apply slightly more force.

Vertical leg or vertical drum support compactors offer bolt leg supports so the drum can carry and balance the frame weight. These machines tend to be shorter in length and have less lateral overhang so operators can work closer to obstructions in the front and rear. “The vertical leg design allows the operator to sit slightly higher, with a better field of vision below,” Locklin says. “Also, the components are located inside the frame and out of harm’s way.”

But on a perimeter frame compactor, components such as the engine and the water tank are mounted outside the frame, which may provide an advantage by making service easier. “Maintenance personnel do not have to work over or under the large horizontal supports,” Sharp says.

Flow troubles?
Renters should also look for a compactor with a reliable water spray system, to keep materials from adhering to the drum during operation. Locklin recommends selecting a roller that has a pressurized dual filtration water system with protected water lines and a quality pump with good electrical connections.

When you call your rental dealer, ask them if the unit they’re proposing to rent to you has easy access to the water spray components so maintenance doesn’t set you back later. Depending on the length of time you rent your compactor, you may need to inspect the lines, clean or change the filters and replace or clean the jet nozzles periodically. “In colder climates, always drain the water from the system when it’s not in use,” Locklin adds.

You can also avoid problems with the water system by using clean water. If the machine’s drum requires more even dispersal of water, Sharp suggests a cocoa mat or a high quality water distribution mat. A cocoa mat has a rough, bristly texture and can be laid over the drum. As water comes down out of the water spray system, it filters through the mat and then distributes evenly over the drum. “This type of mat works well in hot or windy environments,” Sharp explains.

A water distribution mat consists of ribbed rubber. “It fits across the width of the drum, and water collects in the ridges of the mat until it distributes across the drum,” he says. Either of these mats can be used to keep the drum lubricated so asphalt doesn’t stick to it.

Rental checklist

  • Figure out the necessary drum width for your job.
  • How much does the machine weigh and what type of transport vehicle will you need?
  • Note the water tank capacity. How many hours will the supply last?
  • What is the hourly fuel consumption?
  • Recognize the daily service requirements and how long it takes to perform each.
  • Perform a walk-around the machine. Look for wear or damage to the drums. Torn or damaged hoses should be replaced by the rental dealer immediately, and as with all vibratory equipment, hardware has to be tightened from time to time.
  • If you plan to use your rental compactor on asphalt applications, create a pre-production list that includes the tons per hour to be paved, paving width, paving speed, paving thickness and mix design so you can choose a compactor to meet jobsite conditions.

Sources: United Rentals and Caterpillar Global Paving Division.