Forestry equipment strikes fear into trees
Published in: Hydraulics & Pneumatics
Date: 11/1/2000
By: Heney, P J
Forestry applications both In the field and in processing plants – require fluid power’s brute force and exact positioning.
If a tree were to write a horror flick, undoubtedly the serial killer would not be Freddy Krueger, but instead would look a lot like the Tree Mulcher. This attachment for full-size excavators, designed and built by Shinn Systems, Inc., Concord, N.C., “eats trees” where they stand, reducing them to commercial grade mulch – all with a single operator. This equipment can clear sites, utility right of ways, and developments. It is particularly advantageous in wet areas or sites with limited access, where it travels through and clears anything in its path, leaving a bed of mulch on the ground.
Advantages
The Tree Mulcher’s advantages over conventional clearing methods are staggering – one person can clear twice the number of trees per day as a five-person crew with chain saws, chippers, and excavators. The machine operates on rainy days, and in areas where it is impossible for anyone to walk, much less clear vegetation. It can clear trees of any diameter. Compared to clearing with tub grinders, it greatly reduces the number of operations. The crew does not have to:
* push the tree over
* make a windrow (or pile) of trees
* cut the tree into pieces which will fit into the tub
* position the tub grinder on the site
* load the tub grinder, or
* operate the grinder.
The Tree Mulcher grinds a tree into chips where it stands, as opposed to cutting down the tree first, Figure 1. (An optional ripper attachment extracts the stump for grinding.) Other self-propelled grinders require that the trees be cut beforehand, and the stumps must be low enough for the machine to get over them. The Tree Mulcher can grind stumps to 6 in. below the surface as it progresses. It eliminates burning, and after a right of way is cleared, a 4- to 5-in. layer of mulch is left on top of the ground – which proves to be an excellent method of source point erosion control.
Operation
To run the Tree Mulcher – which is driven by its own gasoline or diesel engine – the operator sets the pump at full stroke and the engine at running speed. With the cutter head at operating speed, the front of the cutter shield is placed against the trunk of the tree about 20 ft above the ground. The excavator then walks forward, pushing the cutter head into the trunk, so that the teeth grind into it. The weight of the tree top causes it to fall away from the Tree Mulcher.
The operator then grinds the remaining trunk of the tree down into the ground. The cutter head next is positioned with the push bar on the ground and the tree top is ground by drawing it into the grinder head by moving the head back and forth along the ground.
Hydraulics
Hydraulic power plays an integral part in the performance of the machine. The hydraulic system that operates the cutting tools is completely independent from the excavator’s hydraulics. The power to perform this work is produced at the rear of the excavator (where the counter weight once was), about 30 ft from the end of the excavator boom.
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Fig. 1.
Shinn produces four sizes of the Tree Mulcher, the SC-120, SC-150, SC-220, and SC-300. The three smallest systems use Gold Cup P1 11 P axial-piston pumps and M11 axial-piston motors from Denison Hydraulics, Marysville, Ohio. The pumps incorporate torque limiting that is set for the particular make and model of engine used. Torque-limiting the system prevents shock loads into the engine and drive. Other manufacturers of similar equipment have actually broken crankshafts while trying to reproduce the Tree Mulcher’s performance.
The SC-120 and SC-150 use Cummins 240-hp engines, and the SC220 uses Caterpillar 3306 or Cummins M11-300 engines, which are rated at 315 hp and 300 hp respectively. These engine-pump packages can deliver 95 gpm and 5000 psi. The SC-300 is powered by a 525-hp CAT 3406E that drives a Denison P24P (24 in.3/rev) axial-piston pump and two M11 motors. The SC-300’s system runs at 198 gpm at 5000 psi.
The hydraulic motors drive Poly Chain GT2 belts from Gates Rubber Co., Denver, which, in turn, rotate the cutter drum. The SC-300 system has the ability to regenerate cutter head speed from 0 to 1340 rpm in less than 3.8 sec. The system produces enough torque to grind 10- to 12-in. trees continually without stalling the cutter head. Using a closed-loop hydraulic system allows Shinn to use smaller reservoirs and smaller coolers, an important consideration in mobile applications.
All systems have full instrumentation in the control cab, and are offered with manual or electronic stroke control. The electronic control uses a joystick and a Denison Jupiter 900 driver card. System parameters monitored include: inlet suction, charge pressure, charge filter bypass pressure, system pressure, return pressure, hydraulic fluid temperature (with over-temperature warning), return filter bypass, and fluid level.
The instrumentation package allows troubleshooting most problems over the phone, before having to dispatch a technician.
Short-run hoses result In easier In-field replacement
The Tree Mulcher design was created with functionality in mind. Hoses running down the boom are comprised of short lengths (about 48- to 60-in. long) instead of one continuous hose. Although using one hose would have been cheaper, the nature of the operation resuits in stray limbs or other debris occasionally damaging the hose. The equipment is often located in remote areas, and generally, the operator is the one who replaces damaged hose. (Imagine dragging a 40-ft replacement hose down a right of way, crossing streams, etc.)
Shinn decided that the slightly extra expense of using shorter runs interspersed with bulkheads, Figure 2, was worthwhile. With this setup, the operator can easily remove the damaged section of hose, take it to a hose shop, and get another one made of the same length – or carry a few spares.
On the SC 120, 150, and 220, 1 1/2 in. Gates C13 5000 psi hose (two runs) is used; on the SC 300, four runs are used. The bulkhead features welded stops for the bulkhead fitting – which prevents it from turning while replacing hoses in the field.
A side benefit of using the shorter runs is that there is a reduction of movement during operation. Long hoses tend to straighten out when subjected to pressure, increasing the possibility of chafing against the boom and other components. This is particularly important with the Tree Mulcher, in which the system pressure is constantly varying from 800 to 5000 psi several times per minute.
Other benefits of design
The electronic package makes for increased longevity through ramping the pump up and down. This is very important for this application, because it can experience massive over-running of the engine from the flywheel effect of a 1200-lb. cutter head during shutdowns. The ramp controls take the activation and de-activation of the pump to a consistent level that would be impossible to achieve if left to the operator.
The Denison package has proven to be very reliable in this extreme application. Precise pressure control is a must in a system that can go from 800 psi (cutter head freewheeling) to 5000 psi (cutter head stalled) in a matter of milliseconds, Denison incorporates valuing which relieves the the pressure when the cutter head is stopped and uses a portion of the overpressure to actually drive the displacement control and swashplates back toward zero. This results in an excellent response time of the pump to overpressure – about 70 ms.
The torque-limiting feature also allows the Tree Muncher to use the full power band of the engine. Without this feature, compensator pressure would be set to whatever pressure corresponded to the maximum power output of the engine (i.e. 3850 psi for a 250-hp engine). This forces the machine to waste energy from having to operate at maximum flow and pressure. With the torque limiter, pressure can be set to maximum (5000 psi). As a result, when the cutter head stalls, the pressure immediately increases to maximum, and provides maximum torque to re-accelerate the cutter head. As the motor speed increases at the cutter head, flow through the system increases to the point that the maximum torque output of the engine is achieved and the limiter adjusts the pump to maintain the horsepower limit (250 hp) through the remainder of recovery. A good operator can keep the system on the torque limiter setting, taking advantage of the full power output of the engine.
Problems solved
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Fig. 2.
When Shinn first began building this system, they were using an older model of axial-piston pumps. One of the first problems encountered was pump cavitation during sudden stalls of the cutterhead. This stemmed from locating overpressure relief valves in the motor – on the end of the excavator boom. Because pressure equalizes at 800 psi under no load, during a sudden stall, pressure had to reach 5500 psi before the relief opened to complete the loop. To cure this, Shinn installed accumulators at the front and back, to provide make-up oil.
They also began ordering the units with larger charge pumps. Pressure control proved to be a challenge. The system pressure was set to 3800 psi, but when plotting system pressure, they discovered overshoots to 6500 psi. Explains president Ricky Shinn, “The graph looked like a Richter scale! After we began using the Denison product, we eliminated a lot of the support plumbing on the machine. The new system’s pressure was plotted and the maximum overshoot we have seen is 250 psi – on a system set to 5000 psi.”
Shinn also added accumulators to augment flow and pressure during spike conditions, which has also proven to be beneficial. Pressure spikes usually occur while grinding the stumps. As the cutter teeth grab into the stump, large pieces of wood can be drawn up into the head, which will stop the cutter drum and the motor dead in their tracks. With the pump fully stroked and pumping 95 gpm, a pressure spike can be generated quickly.
At one time, Shinn adjusted the pump control so that the operator could reverse the direction of the cutter head to release trapped wood. When Ricky Shinn went into the field to observe the machine in action, he saw an operator stall the cutter head on a log. The operator proceeded to place the head on the ground, and using the arm (and weight) of the excavator, rubbed the drum on the ground, reversing the direction and dislodging the log (all with the engine at full speed). The log fell out, he picked the cutter head off the ground, and it re-accelerated. Explains Shinn, “He went on like nothing had happened at all. It was there that I realized that my reversing cutter head feature was a waste of time!”
Loader attachment makes tree loading, unloading a breeze
Free Equipment Design, Inc., New Rinegold. Pa.. is the company behind the Tree Boss, a one-man tree handling system for balled trees – larger trees raised in nurseries that have been dug-up, and their root balls wrapped in burlap to keep them intact and ready for transplantation. The Tree Boss is an attachment for skid-steer loaders and used to load and unload these balled trees onto and off of trailers. It can work with a variety of sizes: 20-in. ball trees (180 lb) to 50-in. ball trees (2600 lb). This attachment enables the operator to work with no assistance, reducing labor costs and saving time. One operator can essentially do the job of an entire crew of four workers.
Rotary actuators to the rescue
The attachment uses three hydraulic cylinders and two helical hydraulic rotary actuators. Two cylinders operate arms that open and close to grab the root ball, the third cylinder operates a foot that adjusts to the diameter of the root ball, providing a third contact point when the root ball is grabbed. The rotary actuators used together provide for lifting, rotating, and positioning the tree as it is picked up and loaded. The actuators, Figure 3, from Helac Corp., Enumclaw, Wash., are mounted together with their shafts perpendicular: an HBI-25K with 25,000 in.-lb of torque and 270deg of rotation is mounted vertically, and an HBI42K with 42,000 in.-lb of torque and 1000 of rotation is mounted horizontally. The use of two rotary actuators pro
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Fig. 3.
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Fig. 4.
vides maximum articulation capabilities within the most compact configuration possible, while, at the same time, produces tremendous torque and minimizes the number of components. This permits a more aesthetic design, lowers weight, and reduces assembly costs. Further, this is an application prone to considerable abuse: there is only a short span of time that trees can be transplanted, and the nurseryman’s emphasis is on how many trees are moved in a day – not on babying the equipment.
Helac’s actuators offered several advantages over other rotational devices and actuator designs:
* very high torque output is achieved in ultra-compact configurations
* the shafts incorporate integral large diameter drilled and tapped mounting flanges, which contribute to mounting ease and flexibility
* integral, extra-large-diameter ball bearings support the shaft and enable the actuators to carry heavy radial, moment, and thrust loads without the need for additional external bearing support
* the elimination of all internal bypass and external leakage (and the nearly zero backlash characteristics) produces smooth, positive positioning without drift
* no external moving parts are exposed, and
* the helical gear design offers exceptional resistance to shock loading and abuse.
Hydraulic control
The Tree Boss uses the auxiliary oil supply of the skid steer loader, and is supplied with self-contained hydraulic controls to actuate all functions of the attachment Beiler Hydraulics, a local fullline distributor of hydraulic components and systems, was instrumental in the design and specification of the control package. Features of the system include:
* compact package
* excellent flow and pressure capabilities due to enhanced design of the Nachi directional control valves
* ease of plumbing to the customized Daman manifold, and
* easy setup and troubleshooting, due to indicator lights on the directional control valves.
Eight functions are controlled by the skid steer loader operator: left rotation, right rotation, tip up, tip down, grabber arms open, grabber arms close, foot in (to adjust to diameter of root ball), and foot out. An electronically controlled hydraulic valve mounted inside the boom controls all functions. The operator has access to these functions via switches integral to a handgrip that replaces the original loader’s joystick.
Summary
Any designed would be proud of the fact that were no problems associated with the mechanics or hydraulics in designing this machine. However, one challenge didn surface – there was a marketing problem, as potential customers didn’t believe the attachment could really replace four workers!
AUTHOR_AFFILIATION
Thanks to Ricky Shinn, president of Shinn Cutter Systems, Concord, N.C. and Mark Lehnerz, Gates Rubber Co., Denver, for their assistance.
AUTHOR_AFFILIATION
Thanks to Lee Squires of Tree Equipment Design and Thomas Krause of Helac. Corp., for their assistance.
Copyright Penton Media, Inc. Nov 2000