In tandem for energy-efficient hydrostatic 4-wheel drive

Baljer & Zembrod round wood sorting and transporting carriages with new dual circuit pump from Rexroth

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Round wood sorting and transporting carriages with heavy-duty cranes for tree trunks: Energy efficient yet dynamically speeds up your processes, automatic positioning increases productivity. The latest generation from Baljer & Zembrod includes a completely redesigned hydrostatic 4-wheel drive. It is based on two A30VG tandem axial piston variable pumps, which supply four wheel motors in closed circuits. The vehicle can be steered automatically towards its destination along the track with the proportional electro-hydraulic control.

If you associate the primary processing of timber with labor-intensive manual work, you obviously haven't yet seen Baljer & Zembrod crane and conveyor systems in action. One person can control the round wood yard alone with this machinery. They can travel on a track to the pick-up location from the driver cabin of the round wood sorting carriage. There they can pick up the round wood with the crane and lay them on a special crosscut table. The trunks are measured and trimmed to different lengths. Then the crane picks up the sections and lays them at specific points along the 120 to 140 meter long stretch of tracks. With the new travel drive with the A30VG electronically regulated tandem axial piston variable pump from Rexroth, the carriage now travels to its destinations automatically, increasing productivity.

Baljer & Zembrod is a medium-sized machinery manufacturer with approximately 100 employees in Germany, France, Poland and the Czech Republic, specializing in all things to do with round wood yard machinery: Round wood sorting and transporting carriages, various root reducers, perforated rotor debarking machines, router head debarking machines, stationary cross-cutting stations, sorting lines and many more modules are available.
The company has established itself as the leading European provider within this niche over the past 30 years. It has produced more than 1,800 different kinds of round wood sorting and transporting carriages and has expanded its business to include scrap and recycling applications.
One reason for its success is the extremely flexible modularity of its solutions. The manufacturer produces individual solutions quickly and economically, building designs with different track gauges, power settings and five different crane designs with load capacities of up to 6,000 kg with a reach of 20 meters onto a basic frame.

From series connection to four circuits

Baljer & Zembrod has completely revised its hydrostatic travel drive for its round wood sorting and transporting carriages in recent years and the new design has been in series production since January 2017. "Previously we used a closed circuit that supplied four wheel motors", explains Karl‑Heinz Würtele, who is responsible for Sales and Service at Baljer & Zembrod. Under certain operating conditions, high total pressures could build up between the wheel motors due to series connection. This required significant design safety measures.

"We wanted to find a new solution for this", remembers Karl-Heinz Würtele. "We reviewed all options, and at the beginning of the process were thinking about a purely electrical travel drive, but it quickly became clear that the solution would not be economical." The hydraulic pumps of the round wood carriage are also electrically driven, but a purely electrical drive concept with the same performance would double the current strength from 100 to 200 amps, which would mean that the entire electrical system would have to be redesigned.

300 mm design length supplies two circuits

Baljer & Zembrod instead decided on a modern electrohydraulic solution. The basis of the new travel drive is the Rexroth A30VG axial piston variable pump, a dual circuit variable pump in a swashplate design with integrated boost pump. It is the most compact pump of its kind, with a design length of only 300 mm and a nominal size of 28 (2 x 28 ccm) with clearance. It supplies two synchronously adjustable circuits that are separated from each other in closed circuits with a volume flow or 28 ccm per rotation each.

A double pump with two A30VG pumps is now used for the round wood sorting carriage. This means that four closed circuits, with one circuit per wheel motor are available with the MCR series from Rexroth. This makes a hydrostatic 4-wheel drive with minimal design space realizable.

Hydraulic differential lock

The new concept eliminates the previously occurring total pressures with series connection. The hydraulic differential lock, now easily realizable with the A30VG, prevents spinning of the wheels even with differing wheel loads. Shuttle valves control a pressure sequence valve with the highest pressure as an external pressure cut-off. If the value set is exceeded, the control pressure Ps is connected with the tank.

Significantly increased overall efficiency

Even at an operating pressure of only 170 bar, the drive reaches the same tensile force as previous solutions. "We have achieved significantly higher overall efficiency in this way", says Karl-Heinz Würtele. "The energy efficiency is higher, the components are significantly less stressed and we were able to simplify the design in some areas." Because the pressure is now limited to 170 bar, four-layer hydraulic hoses are now used instead of two-layer ones. During continuous operation, the oil temperature reaches only a moderate 50 °C. "This reduces the effort required for oil cooling and we also save on secondary valves."

Automated travel drive

The new 4-wheel drive significantly improves the driving capabilities of the round wood sorting carriage. In all load scenarios, the carriage accelerates to the maximum speed of up to 190 m/min without spinning wheels. For Karl‑Heinz Würtele, the fact that the travel drive can be automated with the proportional electro-hydraulic control at the pump represents a forward-thinking step. The SPS of the vehicle calculates which round wood belongs to which loading location and controls the corresponding position automatically. The driver then only approves the trip. The automatic drive increases working and energy efficiency because it accelerates and brakes optimally. Compared to manual driving, the automatic drive caps peaks in demand and decreases medium energy consumption.

"We cooperated closely with Bosch Rexroth when we were revising the travel drive", remembers Karl‑Heinz Würtele. "But we did more development in the field with the machine than at our desks." The results speak for themselves: Within a few months, the project was ready for series production and from January 2017 the A30VG with two circuits has been used in all new round wood sorting and transporting carriages from Baljer & Zembrod.