Fig. 2 — In this travel drive, a speed detector reports drum speed to the control unit to compare speeds. If a slipping drum is detected, the control unit activates proportional valves, which operate the dynamic multiple disc brake in the gearbox.

Fig. 3 — Pumps for the vibration drive mounted to the diesel engine power takeoff help save space.

Fig. 4 — This gearbox offers a transmission ratio of 25.00:1 and a 5,000-N-m output torque at the maximum pressure of 400 bar. This is equivalent to a radial-piston motor with 802 cm 3 , which is used for 7- to 9-ton tandem rollers.

Fig. 5 — Travel and vibration drive for tandem roller with split flow pump.


In the past, radial-piston motors with a through drive were practically the only type of motor used on road rollers and pavers with tandem rollers and a split drum. The right configuration of hydraulic motors or the use of a flow divider provided an anti-slip control system. To open up this area of application, Bosch Rexroth AG worked with manufacturers to design a new travel drive concept, Figure 1.

First, a gear was needed that let travel and vibration drives be installed separately for each side, Figure 2. In order to still provide anti-slip control, a speed detector reports drum speed to a Rexroth control unit to compare speeds. If a slipping drum is detected, the control unit activates proportional valves, which operate the dynamic multiple disc brake in the gearbox. Compared with radial-piston arrangements, the benefits of the gear-box and hydraulic motor system include higher efficiency, longer service life, less tubing, and no flow divider. It also offers higher tractive forces because no summation forces act on the hydraulic motors.

With tandem rollers, whether two or four drums are used, the main assumption is that the diesel engine across the drive direction and mounted pumps cannot exceed maximum permitted machine width. This restriction made it impossible to use two independently controllable vibration pumps with half the displacement value, rather than one vibration pump with a vibration valve.

But modifying the housing of a variable-displacement pump with a nominal volume of 18 or 28 cm 3 let the pumps mount directly to the power take-off of a diesel engine, Figure 3. As a result, the travel pump and vibration pump can be used in the traditional compact configuration with a vibration pump at the power takeoff. This eliminates vibration valves, halves the displacement of vibration motors, increases compaction performance thanks to identical or independently controllable frequencies, and prevents unwanted beat frequencies caused by vibration.

Tandem rollers with a split drum also make particular demands on the travel gearbox, Figure 4. These drive systems require the travel and vibration drive to be arranged on the same side. Therefore, Rexroth's gearbox manufacturer developed a planetary gear drive that allows a through drive for the vibration. When designing the gearbox, the priority was to keep the overall drive system as short as possible. Axial-piston motors are used as travel and vibration motors.

The new gearbox consists of a planet stage on the output side and a spur gear stage on the drive side. This means the travel motor can be coaxially offset, and the vibration motor can be coaxially installed. The shaft leading to the vibration apparatus passes through a hole in the drive-side spur gear and the sun wheel of the planet stage. A robust tapered roller bearing supports the weight of the machine like a wheel bearing. The drive unit attaches directly to the drum and is joined to the machine frame via rubber elements.

A speed detector and multiple-disc brake are part of the anti-slip control system. The brake, which is designed for dynamic operation, can also work as a parking brake.

Efficient anti-slip
Another development, the A30VG28 split flow pump, delivers anti-slip control to tandem rollers between 3 to 8 tons, Figure 5.

The volumetric flow of an axial-piston rotary group is divided in half, so the same number of pistons is used for each high-pressure flow. The pump is controlled via one control unit that ensures oil flows are almost identical and, therefore, ideal for driving two synchronously running drums. The equal volumetric flows prevent drum slippage if, for example, the roller has to drive up a ramp onto a truck for transportation.

The closed-loop pump features a swash plate design, split flow in the rotary group, one common suction port, four pressure ports, and maximum pressure to 450 bar.

Sprocket and wheel drives
When designing tracked pavers, the goal is to have the widest possible conveyor within the machine without exceeding permitted transport width. Consequently, only a limited installation width is available for the truck travel gear. To solve this problem, two more gearbox sizes with a short overall

length were developed. The new gearboxes, used with plug-in motors, make it unnecessary to use special gearboxes with spur gear primary stages in compact tracked pavers with a 2.5-m transport width.

A two-speed gearbox for wheeled and tracked pavers with rubber tracks are designed to reach high traveling speeds. This drive unit, used with an axial piston displacement motor, allows a slow working speed with good true-running properties and high traveling speed in fast mode. It involves a three-stage planetary gear, whereby it's possible to change down a gear for fast operation. The gear-changing process is always smooth, regardless of the position of the gearwheels. If the multiplate clutches are depressurized, they act as a parking brake. A high variation range with reduction ratios of 40.5 and 279.5, when used with variable displacement motors, makes traveling speeds of 20 km/h with wheel-mounted pavers and good true-running properties of the hydraulic motor during installation (even at working speeds of 1 m/min) possible.

When used as a travel gearbox in rubber track machines, heavy taper roller bearings are used because of the high radial forces encountered in the field. And last, but not least, short overall length was the goal.

Chipping away with hydraulic drives

The diesel-powered GHM 300 chipper from Forus GmbH provides precise hydraulic pressure control for smooth operation, thereby extending machine life and improving operator comfort.


Since 1999, Germany-based Forus GmbH has been manufacturing wood choppers and recycling machines for materials such as mature timber, domestic and bulk waste, and root stock. The GHM 300 chipper is designed to chop green wood for thermal and material recycling, including brushwood, branches, and summer prunings, as well as tree and shrub prunings and small whole trees. Depending on where the chipper is used, it can be set up on a vehicle trailer, forwarder, or lorry.

The chopping machine consists primarily of a feed system for material delivery, the cutting mechanism complete with chopper rotor, and a discharge unit for the material once chopped. The chopper rotor rotates at 1,000 rpm. A new hydraulic direct drive was designed specifically for this application. The closed-circuit drive consists of a Bosch variable-displacement axial-piston pump and axial-piston motor, driven by a 224-kW powered diesel engine. The pump supplies the hydraulic motor, which is directly connected to the rotor shaft on the chipping unit. A second axial-piston pump operates ancillary functions, such as feed rollers and discharge accelerators.

The advantage of this drive is the balance of operation times in relation to the drive and chopper rotor outputs. The drive makes defined starting and braking possible, which prevents vibrations when running at critical speeds. Pressure in the flow pipe records the changes in the required output. The volumetric flow of the control pump can be steplessly adjusted so it corresponds with the mechanical output required, all while maintaining a constant hydraulic output.

The hydraulic direct drive also forms the basis for the load-dependent feed control also used in the GHM 300. This second operating circuit for the delivery and feed device drive allows a continuous processing sequence and helps prevents pressure peaks.

In addition to the possibilities for open-and closed-loop control, the hydrostatic drive has some advantages over the mechanical drive. Mechanical power transmission elements such as V-belts or propeller shafts are omitted. The hydraulic drive offers more flexible variations in structure because the diesel motor and chopper rotor don't have to be rigidly connected to each other.