Boom Aerial Work Vehicle Upper Arm Hydraulic Cylinder

Extend: Pushing the Jib Up

When the upper arm cylinder extends, it pushes the jib upward against gravity. The full piston area (Φ110 = 9,503 mm²) is available for this push. At 23 MPa, the extend force is approximately 219 kN — more than sufficient to raise the jib, platform, and rated personnel load through the full articulation arc.

Retract: Pulling the Jib Down

When the upper arm cylinder retracts, it pulls the jib downward — with gravity assisting, but against the inertia and wind resistance of the jib section. The retract force comes from the annular area (bore area minus rod area). With the Φ75 rod, the annular area is 5,085 mm², producing a retract force of approximately 117 kN at 23 MPa. If the rod were Φ80 (like the lower arm), the annular area would drop to 4,476 mm² — a 12% reduction in retract force that would slow jib descent and reduce the operator's downward positioning control.

The lower arm raises the boom above the wall height. The upper arm cylinder then extends to angle the jib downward over the wall, positioning the platform on the building side. This is the most common up-and-over manoeuvre — used for rooftop HVAC maintenance, window installation on set-back floors, and building facade work above overhanging structures.

The lower arm raises the boom to the underside of the overhang. The upper arm cylinder retracts to fold the jib horizontally, sliding the platform underneath the structure. This is used for bridge inspection, canopy maintenance, and industrial pipeline access where the work area is below and behind the structural element.

The lower arm extends horizontally to position the knuckle above the target area. The upper arm cylinder then extends to lower the jib vertically downward into the space — like lowering a platform into a pit, a shaft, or an atrium from above. The 815 mm stroke allows the jib to articulate through approximately 135° of total travel at the knuckle — from fully elevated to near-vertical descent.

Can I interchange the upper arm cylinder and the lower arm cylinder?

No. Despite sharing the same GTHZ210C platform designation and the same 2-Φ9 port style, the two cylinders have different bore, rod, stroke, pressure rating, and installation distance. The upper arm cylinder (Φ110/Φ75/815 mm) is dimensionally incompatible with the lower arm mounting (which requires Φ100/Φ80/1,458 mm). Even if a physical fit were possible, the different rod-to-bore ratios would produce incorrect force characteristics — the upper arm cylinder's lower buckling resistance would be dangerous in the lower arm position.

The jib descends faster than it rises — is this normal?

Yes — within limits. The retract annular area (5,085 mm²) is smaller than the extend full-bore area (9,503 mm²). At the same flow rate, the retract stroke moves faster because the same volume of oil fills a smaller area. Additionally, gravity assists the downward jib movement. The combination makes the jib descend approximately 50–80% faster than it ascends. This is a designed characteristic, not a fault. However, if the descent speed has increased noticeably compared to when the machine was newer, the upper arm cylinder may have internal seal bypass allowing oil to bypass the piston during extension — check by performing a hold test at mid-jib angle under load.

Should I replace the upper arm cylinder and lower arm cylinder at the same time?

Not necessarily. Unlike forklift tilt cylinders that must be replaced as a matched pair for symmetry, the upper and lower arm cylinders are independent actuators serving different joints. They wear at different rates (the upper arm cycles more frequently and typically fails first). Replace each cylinder on its own failure timeline. However, if both cylinders are showing symptoms simultaneously and the boom lift is scheduled for a major service, replacing both during the same downtime event saves the labour cost of a separate boom teardown.

Why is the upper arm cylinder's working pressure 23 MPa — the highest on the boom lift?

The upper arm cylinder operates at the highest system pressure because it must provide adequate retract force from a reduced annular area. On the extend stroke, the full Φ110 bore at 21 MPa would produce 200 kN — already sufficient. But on the retract stroke, the annular area at 21 MPa would produce only 107 kN. The additional 2 MPa (23 MPa total) raises the retract force to 117 kN, ensuring the jib can be pulled down smoothly even when wind or inertia oppose the movement. Contact Korea Ever-Power for a replacement quotation.

Replaced the upper arm cylinder on our GTHZ210C articulating boom after the original started drifting at the knuckle — the jib would slowly drop about 5° over 10 minutes when we were doing facade work at 18 metres. Extremely unsettling for the guys in the basket. The Ever-Power replacement eliminated the drift completely. Jib now holds rock-solid at any angle. The Φ110 bore is impressive — this is a big, serious cylinder. Needed a forklift to unload it from the truck. Installation took about 3 hours including bleeding the circuit. Back in service and working perfectly.

We specialise in high-rise building maintenance in Auckland using articulating boom lifts. The upper arm cylinder fails before the lower arm on every machine in our fleet — we estimate roughly 2:1 replacement ratio. The Ever-Power GTHZ210C-620500-000 gives us a viable alternative to the OEM channel, which in New Zealand means 8–12 week shipping from Europe or the US. Ever-Power delivers to Auckland in about 4 weeks, and the price difference funds the air freight if we need to expedite. Two units installed, both performing well after 5 months of daily use.

Functionally excellent. The jib articulation is smooth and precise — better than the original cylinder at 6,000 hours. My only concern is cosmetic: the paint finish on the barrel was slightly uneven in one area, with a visible run mark. It doesn't affect performance or durability, but on a 100 kg cylinder that costs this much, the finish should be consistent. I'd give five stars if the paint quality matched the machining quality, which is outstanding.

We ordered the upper arm and lower arm cylinders together for a complete boom rebuild at 10,000 hours. Both arrived in one shipment, 26 days to Jeddah. Our technician installed both in a single day — having both cylinders ready at the same time meant we only needed to crane-support the boom once instead of twice. Combined cost was about 45% of the OEM equivalent for both cylinders. The boom lift has been back in service for 4 months with zero issues on either cylinder.