VIBRATORY ROLLERS

Vibratory rollers provide compactive force by a combination of weight and vibration of their steel compaction rolls, commonly referred to as drums. Those used for compaction are self-propelled and vary in weight from 7 to 17 tons.

Propulsion for single-drum models is provided by pneumatic-tired wheels, as shown in figure 11-39. The

Figure 11-39.-Vibratory roller.

drums on vibratory rollers vary from 3 to 5 feet in diameter and 4 to 8 feet in width.

The engine, providing power for propulsion, also powers the hydraulically driven vibrating unit. Vibrations are generated by a rotating eccentric weight inside the drum, the speed of which determines the frequency, or vibrations per minute, of the drum. The weight and distance from the shaft of the eccentric determine the amplitude (amount) of the impact force. Both the frequency and amplitude of vibrations are controlled independently of roller travel and engine speed.

The vibration frequency of rollers used for compaction is generally between 2,000 to 3,000 vibrations per minute (vpm), depending on the model and manufacturer. Some models provide only one or two specific frequency settings; while others may provide a full range of frequencies within certain limits; for instance, 1,800 to 2,400 vpm.

Vibratory rollers achieve compaction through a combination of three factors: (1) weight, (2) impact forces (roller vibration), and (3) vibration response in the soil.

Weight

Weight is the natural force in compaction of soils. Vibrating rollers amplify their static weight through vibration to increase the overall dynamic weight.

Impact Forces

The impact forces are those generated by vibration of the compaction drum. They are regulated by controlling the frequency and amplitude of the vibration. The amount of impact force required to obtain optimum density depends on the type of material being compacted. The impact forces also vary with the diameter of the drum and the width and the ratio of the roller static weight and dynamic (impact) force.

Vibration Response

The vibration response in the soil or material is the result of the way the forces are exerted upon it by the vibratory roller. As with other types of rollers, the material will compact easily or with difficulty, depending on its moisture content, cohesion characteristic, particle shape and texture, and confinement; for example, sandy soil requires more vibration and less impact force (amplitude). However, a soil with higher clay content requires more amplitude than vibration because of the kneading action necessary to compact the clay. Vibratory rollers exert repetitive dynamic force on the material, rather than the static force used by other rollers.

The frequency and roller speed should be matched, so there will be at least 10 downward impacts per foot of travel of the roller. The speed of the roller increases for a given frequency of vibration, and the spacing of the impacts grows farther apart.

When using vibratory equipment, keep in mind that the energy imparted by the vibratory wheel must be absorbed in the material being compacted. Controlling the amplitude permits the operator to vary the force developed from the wheel and, therefore, the energy imparted to the material. A change in the lift thickness and material gradation content may require adjustment in the amplitudes being used.

NOTE: It is important that the roller vibrates only when it is moving. If vibration continues while the roller is standing still or changing direction, the vibrating drum will leave an indentation in the material at the stopping point.

Most modern rollers have automatic cutoffs for vibration when the roller stops moving.

The rollers used in the Naval Construction Force (NCF) are equipped with two interchangeable drums. One is known as the sheepsfoot, as shown in figure 11-40; and the other is known as a smooth drum, as shown in figure 11-41.