Case Study
Case Lifter Study
A scissor lift that passes every structural check, with one behavioral risk left to close.
The in-depth structural analysis of the case lifter: real axles, a solid arm section, and a genuine bottom-of-stroke pose, so the whole envelope checks out, leaving cylinder synchronization as the sole open item.
Verdict: every structural check passes across the entire travel envelope. The one open risk is behavioral, not a part: two independent cylinders with no synchronization.
Scope
The same 1000 kg payload target (plus a roughly 164 kg platform, safety factor 1.5 on working weight), the same mild-steel assumption (yield 250 MPa, shear yield 145 MPa), and the same static method as the shaft study. This machine is a single-stage scissor with one hydraulic cylinder per side. Actuation sizing is out of scope; the force demand is stated for the vendor.
Force required to lift
Same mechanics as the shaft lifter, from a worse angle. At 5.23 degrees the multiplier is 10.92x, so the 17.13 kN design load demands 187.1 kN of total horizontal push, or 93.6 kN per side, falling to 60.6 kN total at the 15.8 degree top. Because the modeled pose is the true bottom of stroke, this worst case is the floor, not a hidden low-angle trap.
Geometry and derivation at the 5.23 degree floor.
The joint reaction map
With one pusher per side, each side frame is its own bottom-driven load path. At design load the main hinge sees 93.7 kN resultant, the center pivot 93.6 kN, and the roller verticals 4.28 kN per side. As on the shaft lifter, the top hinges carry no horizontal force by platform equilibrium.
Reactions per side at design load.
Structural checks
| Check | Result | Verdict |
|---|---|---|
| Main hinge axles (25 mm, double shear, with retention) | 95 MPa (66%); overstress floor 3.4 deg, below the reachable range | Pass everywhere in the envelope |
| Center pivot (30 mm load-bearing bushing) | 66 MPa (46%) double shear; 132 MPa (91%) even single shear | Pass |
| Arm bending (solid 25 by 70 mm arms, 30 mm hole) | 169 MPa, 68% of yield at the worst pose | Pass |
| Rollers (ball-bearing) and pusher block | 4.28 kN per side, 38% of static rating; block strong by inspection | Pass |
| Travel envelope and synchronization | 5.23 to 15.8 deg, modeled pose = full retraction; two independent cylinders | Envelope clean; sync is the open risk |
Axle and bushing shear
The main hinge is a 25 mm axle in double shear, confirmed by section, with retention screw holes. At 93.7 kN it runs 95 MPa, 66 percent of allowable, and its overstress floor of 3.4 degrees sits below anything the 50 mm stroke can reach, so no reachable pose overloads it. On the breakage envelope, the main hinges deform at roughly 2.5 tonnes of payload at the working pose and rupture between about 4.2 and 5.9 tonnes.
Axle and bushing shear, with the breakage envelope.
Arm bending
The arms are solid plates, 25 mm thick and 70 mm deep, with a 30 mm hole at the crossing. The net section modulus against the 3.18 kN.m center moment gives 169 MPa, 68 percent of yield, at the worst pose. A solid 25 mm arm has ample axial and buckling margin on top of that at these loads. This is the structural difference that engineers out the bent-bar failure mode of a thinner design.
The arm section and the bending check.
The travel envelope and the open risk
The modeled pose is full retraction, so the envelope is exactly 5.23 to 15.8 degrees and everything in it passes. The one open structural risk is behavioral: two independent cylinders with no flow divider, sync valve, or mechanical tie. If one side leads, the platform racks, the lagging side's hinge sees more than its half of the load, and the arms take out-of-plane twist that the 2D checks do not cover. At 66 to 68 percent utilizations the structure has real margin to absorb moderate imbalance, but synchronization hardware is cheap and the loads here are 93 kN a side.
The travel envelope at true scale; the modeled pose is the bottom of stroke.
Force demand across the envelope, for the actuation vendor.
Recommendations
- Add cylinder synchronization: a flow divider, a rephasing circuit, or a mechanical tie between sides. This is the only open structural risk on this machine.
- Verify the center pivot's double-shear construction next time the joint is opened; the 91 percent single-shear reading is the thinnest margin on the machine.
- Confirm material certificates at build; all margins assume mild steel.
Assumptions and limitations
- Material grade is unconfirmed; all stresses assume mild steel at a yield of 250 MPa.
- The only safety factor is 1.5 on the working weight, applied to the load. The 145 MPa shear figure is the shear yield, not a further-reduced allowable.
- Static analysis only: no dynamic or impact factor, no fatigue, and no stress concentration at holes. Both sides are assumed to share load equally except where the unsynchronized cylinders are flagged.
- Deformation and rupture loads are isolated pin-shear values on assumed steel; other components govern well below them.
- The center pivot is checked as steel shear only. The bushing material's own bearing pressure limit was not evaluated (projected pressure roughly 104 to 125 MPa); flagged for follow-up with the bushing specification.