Boom Aerial Work Vehicle Lower Arm Hydraulic Cylinder

Induction-Hardened + Hard Chrome

The Φ80 rod is C45 steel, induction-hardened to HRC 55–62 across the surface layer, then hard-chrome plated to 20 μm minimum. On a lower arm cylinder that operates outdoors on construction sites — exposed to rain, mud splatter, abrasive dust, and occasional impact from swinging loads — the chrome layer is the primary defense against rod surface damage. Any scratch or pit in the chrome that reaches the steel substrate will corrode and destroy the rod seal within weeks. The 20 μm chrome thickness provides the wear reserve needed for 6,000–10,000 hours of outdoor operation.

2-Φ9 Banjo Ports — High Flow Capacity

The lower arm cylinder uses 2-Φ9 banjo-style port connections rather than threaded ports. Banjo fittings allow the hose to connect at any rotational angle around the port axis — critical on the lower arm cylinder because the hose routing must accommodate the cylinder's angular movement as the boom rotates. The Φ9 port bore provides the flow cross-section needed for the high oil volume that fills and empties the Φ100 bore during boom raise and lower cycles. At typical boom speeds, the lower arm circuit flows 30–50 L/min — far more than the 5–10 L/min through the steering or floating circuits.

Step 1: Support the boom

Fully lower and retract the boom. Place timber cribbing or steel trestles under the boom to support its weight independently of the lower arm cylinder. Verify the boom is stable before disconnecting any hydraulic lines. On a boom lift, the boom assembly is heavy — it must be safely supported before the cylinder that holds it is removed.

Step 2: Disconnect and extract

Disconnect the two banjo fittings and cap all ports immediately. Remove the pin retainers at both ends of the cylinder. Use a crane sling or hoist rated for at least 200 kg (cylinder weight plus tooling) to support the cylinder before removing the final pin. Extract the cylinder from the turntable — this may require rotating the turntable to align the cylinder with the extraction opening.

Step 3: Install and commission

Hoist the new GTHZ210C-620400-000 lower arm cylinder into position. Align the pin bores and insert both mounting pins. Reconnect the banjo fittings — torque to specification. Bleed the boom circuit by cycling the boom slowly from stowed to approximately 30° elevation, 5–10 times. Check the hydraulic fluid level after bleeding — the Φ100 × 1,458 mm bore holds approximately 11.4 litres of oil that must be displaced from the reservoir. Total replacement time: 4–8 hours depending on machine access and crane availability.

The boom drifts down slowly when holding at angle — is this always the lower arm cylinder?

Not always. Boom drift can originate from the lower arm cylinder (internal piston seal bypass), from the holding valve (counterbalance valve not fully seating), or from the control valve (spool not centring properly). The diagnostic procedure is to cap the cylinder hoses at the holding valve and repeat the drift test. If the boom holds with the hoses capped, the leak is upstream (valve or control circuit). If it still drifts, the lower arm cylinder has internal bypass and needs replacement.

How long does a boom lift lower arm cylinder typically last?

Under normal operating conditions, 5,000–10,000 hours. The lower arm cylinder cycles less frequently than the steering or floating cylinders (the boom is raised and lowered a few times per work shift, not continuously), but each cycle involves very high forces and the rod is exposed to outdoor contamination for extended periods. The primary wear mechanism is not cycle count but environmental exposure — construction dust, rain, and UV degradation of the rod seal over years of outdoor service.

Can I rebuild the lower arm cylinder instead of replacing it?

If the bore and rod surfaces are in good condition, yes — but the economics are different at this cylinder size. A seal kit for a Φ100 bore cylinder costs significantly more than for smaller cylinders, and the disassembly and reassembly of a 137 kg cylinder requires workshop hoist equipment and experienced technicians. If the bore is scored or the rod chrome is damaged, the re-machining cost (re-honing a Φ100 bore + re-chroming a Φ80 rod) can approach 60–70% of a new replacement cylinder. Contact Korea Ever-Power for a new cylinder quotation before committing to a rebuild.

What equipment do I need to replace the lower arm cylinder on site?

At minimum: a mobile crane or overhead hoist rated for 200+ kg lift capacity, a sling set rated for the cylinder weight, timber cribbing or steel trestles to support the boom during removal, hydraulic hose caps or plugs, and standard hydraulic tools (torque wrench for banjo fittings, pin punch set for clevis pins). Two technicians minimum — one to guide the cylinder and one to operate the crane. Allow 4–8 hours total including bleeding and commissioning. If workshop access is available, the replacement is faster because the boom lift can be positioned under a fixed overhead crane.

This was the biggest hydraulic cylinder I've ever handled — 137 kg is no joke. Needed a 2-tonne overhead crane to install it. But the quality is excellent: the Φ80 rod chrome was flawless, the banjo ports threaded cleanly, and the pin bores aligned perfectly with the turntable mountings on our GTHZ210C boom lift. We ran a full drift test — 450 kg on the platform, boom at 45°, held for 15 minutes with zero measurable descent. The old cylinder was leaking about 2° per hour at the same load. Massive improvement. Delivery took 4 weeks but for a cylinder this size that's reasonable.

We service boom lifts for construction and industrial access across Germany. The lower arm cylinder is our most expensive single-item replacement — the OEM quotes €4,500+ with 10-week lead time. The Ever-Power replacement was approximately 40% less and arrived in 25 days. We've installed 3 units on different boom models over the past 6 months, all performing at OEM level. The 25 MPa proof test certificate that comes with each cylinder satisfies our TÜV inspection requirements. This is now our standard source for lower arm cylinder replacements.

Good cylinder, correct dimensions, works well. Took one star for the packaging — a 137 kg cylinder arrived on a pallet with strapping, which is fine, but the rod end was protected with only a plastic cap and some tape. For a cylinder this heavy, the rod cap should be a bolted metal protection plate. The rod chrome was undamaged, so no real harm done, but during shipping a 137 kg cylinder bouncing on a pallet could easily damage an exposed rod end. Suggest upgrading the rod protection for heavy cylinders.

We tried rebuilding a lower arm cylinder in-house once — never again. Re-honing a Φ100 bore and re-chroming a Φ80 rod cost us more in machining fees than the Ever-Power replacement costs complete. Plus we had the cylinder out of service for 5 weeks during the rebuild. Now we order the full replacement from Ever-Power, install it in a day, and send the old one for scrap metal credit. Faster, cheaper, and we get a warranty. Third lower arm cylinder we've bought from them — consistent quality every time.