Landing Leg Screw Jack Assembly

Function

Each screw jack converts rotary crank input into controlled vertical linear travel, raising or lowering the chassis corner to which it is attached. The self-locking trapezoidal thread profile holds any set position without back-driving under sustained load, eliminating the need for separate mechanical lock pins during normal operation. Precise levelling of the plant directly affects burner combustion efficiency, aggregate distribution across the drum cross-section, weigh-bridge batch repeatability, and the geometry of aggregate conveyors — all of which influence both mix quality and sustained output in tonnes per hour. An out-of-level chassis causes uneven aggregate loading, reduces weigh-batch accuracy, and accelerates wear on conveyor idlers, belt edges, and drum riding rings. The robust screw-jack mechanism is sized to withstand not only the static plant dead weight but also the intermittent dynamic impulse loads generated during aggregate charging, drum rotation, and vibrating-screen operation — ensuring the chassis remains stable and correctly positioned throughout a continuous production shift.

Construction & quality

The outer tube is fabricated from S355J2 structural steel square hollow section (wall thickness ≥ 8 mm), plasma-cut to length, welded to the upper mounting gusset plate, and stress-relieved after fabrication before surface treatment. The inner sliding tube is precision-sized to the outer with a working diametral clearance of 0.3–0.6 mm and internally coated to resist rust pack in the annular gap. The screw shaft is machined from 42CrMo4 (4140) medium-carbon alloy steel bar; the trapezoidal thread form (Tr 80 × 10) is single-point turned then induction-hardened across the full thread length to 45–52 HRC and ground to a 7e tolerance class for smooth, low-friction travel under rated load. The travelling nut, keyed into the inner tube, is cast from CuSn8 tin bronze — a material that provides a self-lubricating, sacrificial interface with the hardened screw shaft and can be replaced independently without replacing the whole assembly. The 300 × 300 × 20 mm structural steel base plate is full-penetration fillet-welded to the inner tube foot and drilled on a 200 mm bolt circle for optional ground anchor bolting on weak subgrades. All external steel surfaces receive a zinc-phosphate conversion primer followed by a high-build two-part epoxy topcoat. A grease nipple on the screw boss provides access for in-situ relubrication under load without disassembly.

When to replace

• Noticeably increased manual effort required to raise or lower the leg under load, indicating thread wear, dry-running bronze nut, or corrosion on the screw shaft flanks
• Visible scoring, pitting, or orange-rust corrosion on the exposed thread form of the screw shaft, particularly on sections that remain extended during prolonged on-site operation
• Lateral rocking or slop of the inner tube within the outer section — clearance grown beyond approximately 1.5 mm — indicating worn tube guide surfaces or a deformed inner tube from an overload event
• Cracked weld seams, bent base plate, or deformed inner tube after the leg has contacted a hard obstruction or been overloaded during uneven terrain deployment
• Bronze nut grease purging brown or black contamination, indicating water ingress or abrasive fine material has entered the screw-nut interface