Hydraulic Cylinders for Harvesters, Forwarders and Timber Cranes: Ponsse, John Deere, Komatsu and Others Through a Cylinder Maker's Eyes

Enquiries for cylinders for forestry machines reach Hydroforce more and more often. Customers ask for crane and head cylinders, and also for steering and blade cylinders on the chassis. We build each of them in our own workshop, to the customer’s drawing or from dimensions taken off the old cylinder. Below we go through common machine classes and their cylinders, and show what we design in before machining starts.

What Each Machine Class Carries

Forestry machine builders rarely publish cylinder sizes, and their parts catalogues open through dealers by serial number. The machine layout is public, though, and it shows which cylinders the machine carries.

Size Ranges by Cylinder Type

Sizes follow the ISO 3320 series. Cylinders outside the series we build to the customer’s drawing or from dimensions taken off the old cylinder. The table shows the range of hydraulic cylinders we can build, not the cylinder sizes of a particular machine.

According to published data, timber cranes work at 185 to 270 bar, and harvester heads are rated up to 350 bar.

Crane Cylinders

The lift cylinder sits under the main boom, on the side where the head handles the stem, and a log or the head itself can hit it there. On the Epsilon M120Z the extension cylinder and hoses run inside the boom. On an open crane the barrel will take blows and the rod will get scratched, and we allow for this in the design.

When the boom slews, it also loads the cylinder from the side and wears the rod guide on one side. On long strokes we lengthen the guide and add a stop tube, so piston and guide stay far enough apart at full extension. Extension cylinders are the slimmest on the crane. With a 50 mm bore and a 2500 mm stroke, rod straightness limits the load such a cylinder can carry.

Harvester Head Cylinders

A head closes its knives around the stem and presses the feed rollers against the bark many times for every tree. Cylinders drive the moving knives and the feed arms. A large head takes 320-360 l/min. The delimbing knives carry high loads, so their pivots run in bushings with grease nipples.

Every time the knives close, a pressure spike hits the cylinder. We fit a buffer seal ahead of the rod seal and a spherical bearing in the rod eye, so the cylinder does not bend when the head frame flexes under load.

Chassis Cylinders

Steering cylinders articulate the frame, and blade cylinders raise and lower the blade. On some harvesters proportionally controlled cylinders oscillate the frame at the joint. Many forwarders also have cylinders for cab damping and cab levelling.

These cylinders sit low, where mud and stones fly. The load on a steering cylinder reverses at every turn. Levelling and oscillation cylinders spend most of their time making short strokes around one position, so the rod wears along a short band.

Six Things We Build Into Forestry Cylinders

The solutions below have become standard, both in our workshop and across the industry. A special version for a particular machine we build on the customer’s request.

1. Wipers and Rod Boots

Bark, resin, sawdust and ice all reach the wiper, and a pressure washer drives fine grit into its lip. A common snap-in wiper leaves a small gap at the housing, and slurry collects and freezes in that gap. We therefore prefer a wiper whose outer lip covers the housing, or a metal-cased wiper pressed into the gland.

Not every wiper can clear ice. Many polyurethane wipers work below 0 °C but are not designed to cut ice off the rod. If the machine stands outdoors overnight in winter, we fit its cylinders with a wiper designed to cut ice. Before painting we mask the rod, the wiper, the ports and the grease nipples.

On lift and extension cylinders with a long exposed rod we propose a bellows boot. Dust, bark and blows from branches then land on the boot instead of the chromed rod. The boot material stays flexible in frost. Clamps hold the boot at the gland and at the rod eye, and condensate escapes through a drain or vent hole.

2. Bushings, Eyes and Joints

Side load scores the rod guide, and the wear particles destroy seals faster than outside dirt. We use bronze-filled guide rings on the piston and in the gland.

Drawings for forestry cylinders call for spherical plain bearings from GE 35 ES to GE 60 ES in the eyes. Sand packs into the grease grooves and wears the bronze away, and then the pin starts to score the eye. Once there is free play, every load reversal becomes an impact. Where a joint is hard to reach with a grease gun, we propose a bearing sealed on both sides.

Rods with a spade or eye head we forge in one piece, with no weld and no thread between rod and head. One such rod is shown in our article on a 350 bar cylinder with a solid forged rod.

3. Seals and Fluids

Some machine builders specify a seal kit that may not be replaced. We then machine the grooves on the piston and in the gland to that kit and design the rest of the cylinder around them. In service, spare seals from the same kit fit straight in.

Polyether polyurethane works down to -40 °C, polyester grades down to -30 °C. FKM as a material takes heat up to +200 °C but loses elasticity below -20 °C. Many forestry machines run on biodegradable synthetic ester, and in ester the same seal material has lower limits than in mineral oil. Polyester polyurethane is the most sensitive. For ester circuits we choose polyether polyurethane or FKM and ask the customer for the exact oil grade.

4. Heavy Work

A forestry cylinder works outdoors for its whole service life, and branches and stones scratch it. A thin coat starts to rust at the first deep scratch. Drawings for forestry cylinders ask for a primer and topcoat system from 80 to 150 micron and more in total, and some call for two-component paint rated for 500 hours of salt spray. We apply thick-film coatings and check dry film thickness with a gauge on finished barrels. How we harden and chrome the rods is described in our article on piston rods.

We engrave the serial number before painting, so it stays legible under the coat.

5. Cushioning and Pressure Peaks

Crane and head circuits on these machines carry high flow: large heads take 320-360 l/min. At that flow the piston reaches the end of stroke at high speed and strikes the end cap hard, and through it the crane structure. We therefore fit end cushioning: at the end of stroke a plunger enters a bore in the end cap and pushes the remaining oil out through a restriction. The drawing sets whether it works on retraction, extension or both. We size the restriction for the flow the customer states.

The buffer seal takes the peaks. It must let a thin oil film through towards the rod seal, otherwise pressure is trapped between the two seals and the rod starts to stick and slip. We tighten the piston to the specified torque and lock it, so vibration cannot loosen it.

6. Cold and Heat

In the cold the oil thickens a lot, and head actuators become slow and jerky. Oil standing in long lines stays cold longest. Komatsu fits the 845 with a diesel heater for hydraulic oil. Even so, the first strokes on a frosty morning run on thick oil at high pressure, while the seals are still stiff from the cold.

The lowest ambient temperature in the forestry drawings we receive is -40 °C. We choose the seal material for the temperature range of each drawing rather than one material for every cylinder. In summer the risk is overheating, and a head circuit on ester fluid is where we switch to FKM.

Marking and Documents

Our steel comes with EN 10204 3.1 certificates. The engraved serial number links each cylinder to these documents. On the customer’s request we provide information on the progress of production.

What to Send Us

• the drawing, or the old cylinder so we can take its dimensions;
• machine make, model and mounting position of the cylinder (crane lift, steering, head tilt);
• working pressure and oil flow to this function;
• the seal kit, if it is fixed;
• oil type: mineral or biodegradable ester;
• ambient temperature range;
• paint system, colour and marking;
• quantity per type.

The solutions described above are part of our standard design. We build special versions on request.

Send your enquiry to office@hydroforce.ee or use the contact page.