Key takeaways
- A 16 kN suspended load over two stiles gives about 5.7 MPa in a 100 × 50 × 5 RHS, so specify the section for stiffness and buckling, not for yield.
- A 120 × 80 × 6 mm crosshead over a 1,300 mm span deflects 0.81 mm at 16 kN, which passes a span/1,000 limit of 1.3 mm with little to spare once the rope hitch is welded on.
- Hold stile verticality to 1 mm/m and the frame diagonal to ±2 mm over 2,000 mm; a sling that is square on the floor can still lean in the shaft.
- Keep holes 30 mm from a bend line on 5 mm plate (R + V/2 + t at a 40 mm V die), and cut guide-shoe holes 2 mm undersize for drilling after welding.
- Two powder-coated faces add 0.14 mm to every bolted joint, so mask the guide-shoe faces or plan a shim stack in 0.5 mm steps.
Lift sling assembly design is a stiffness problem wearing the clothes of a strength problem. A 1,000 kg car with a 630 kg rated load hangs 16 kN from the crosshead, split over two stiles. In a 100 × 50 × 5 mm RHS that is about 5.7 MPa of compression, against a yield above 250 MPa. Nothing in the sling is close to yielding, yet slings still arrive on site out of plumb, with guide shoes that will not clear the rail, and with a car that rubs on one side.
The reason is that the sling is an alignment structure. It sets the distance between the guide shoes, the plane they run in, and the position of the rope hitch relative to the car. Those are millimetre quantities, and they are decided by bend radii, weld sequence, fixture design and coating thickness, not by the plate grade.
This guide covers the numbers a sling drawing should carry: crosshead deflection, stile verticality, hole-to-bend distance, weld access, coating allowance and the inspection that proves the assembly before it ships.
What does a lift sling actually have to carry?
It carries the car, the rated load and its own weight, and it carries them in compression through the stiles while the crosshead transfers the rope loads into those stiles.
The load path is short. Ropes terminate on a hitch plate at the crosshead, the crosshead spans between the two stiles, and each stile runs down past the car to the bottom plank or underslung beam. The guide shoes are bolted to brackets on the stiles and are the only contact with the rail.
Two checks matter more than the rest:
- Crosshead deflection under rated load, because it changes the geometry between the rope hitch and the car and can tilt the guide shoes.
- Stile buckling, because a slender RHS in compression fails by buckling long before it yields. The 100 × 50 × 5 section has a second moment of about 1.74 × 10⁶ mm⁴, and over a 3,000 mm length the Euler load is roughly 392 kN against an 8 kN service load, a factor of about 49.
The strength numbers are so far from governing that a bigger section buys nothing except mass and cost. What a bigger section does buy is stiffness, and that is worth paying for only where the deflection calculation says so.
Crosshead deflection, worked through
Take a 120 × 80 × 6 mm box crosshead spanning 1,300 mm between stile centres, carrying 16 kN as a central point load. The second moment is:
I = (80 × 120³ − 68 × 108³) / 12 = 4.38 × 10⁶ mm⁴
Deflection at mid-span, simply supported:
δ = P L³ / (48 E I) = 16,000 × 1,300³ / (48 × 206,000 × 4.38 × 10⁶) = 0.81 mm
That is span/1,600, comfortably inside a span/1,000 limit of 1.3 mm. Change the crosshead to a 100 × 50 × 5 RHS and the second moment drops to about 1.74 × 10⁶ mm⁴, the deflection rises to about 2.0 mm, and the sling is now at span/650, which is where guide shoe noise begins.
Two practical notes on the arithmetic. The ropes rarely land at mid-span, so the real deflection is lower than the central-load case; treating it as central is conservative and quick. And the calculation assumes the crosshead is a plain beam, which it is not once a hitch plate and two gussets are welded to it. A gusset at the rope line is worth more than 2 mm of extra plate, because it shortens the effective span.
Lift sling assembly design: dimensions and the limits we hold
The table covers a 1,000 kg car sling in E250 with RHS stiles, a boxed crosshead and bolted guide shoes. Values in the third column are what we will hold on a drawing; anything tighter needs a conversation about the fixture.
| Feature | Typical drawing callout | Ganesh limit or note |
|---|---|---|
| Stile section | 100 × 50 × 5 RHS | 100 × 50 × 5 to 120 × 60 × 5; 6 mm where the shoe bracket wraps the stile |
| Crosshead section | 120 × 80 × 6 box | 6–8 mm plate; boxed or formed channel |
| Crosshead deflection | span/1,000 | 0.81 mm at 1,300 mm span and 16 kN; span/800 needs review |
| Stile spacing | 1,100 mm | ±1.0 mm between stile inner faces |
| Frame diagonal | equal within 2 mm | ±2 mm over a 2,000 mm diagonal, measured standing |
| Stile verticality | 1 mm/m | 1 mm/m, maximum 2 mm over a 2,000 mm stile |
| Guide shoe face | coplanar within 0.3 mm | 0.3 mm across the pair; 0.5 mm across the frame |
| Bend radius, 5 mm plate | R5 | R5 minimum at 1.0t; R8 preferred at the crosshead corners |
| Hole-to-bend distance | not usually stated | 30 mm on 5 mm plate at a 40 mm V die, using R + V/2 + t |
| Guide-shoe holes | Ø14 for M10 | Cut Ø12, drill after welding; ±0.3 mm on position |
| Weld leg | 5 mm | Continuous at the crosshead-to-stile joint; 15 mm torch access |
| Coating | 70 µm polyester | 80 µm in a damp shaft; guide-shoe faces and threads masked |
| Sling mass | not usually stated | 210–260 kg at this size, depending on stile length |
Bend radii and hole placement on a sling
Sling plate is 5–8 mm, and the bend rules are the ordinary ones applied without exception. On 5 mm E250 the inside radius should be R5, and R8 at the crosshead corners where the bend line runs across the plate width and the outer skin does the work.
Holes are where sling drawings go wrong. A guide-shoe bracket has four to six holes, usually slotted for adjustment, and they sit close to the bend that forms the bracket. The minimum distance from the bend line to the hole edge is R + V/2 + t, where V is the die opening. On 5 mm plate with a 40 mm V die and an R5 bend that is 5 + 20 + 5 = 30 mm. At 15 mm the hole pulls oval and the slot closes on one side, which the installer discovers when the shoe will not sit flat.
Two DFM moves remove most of this risk:
- Drill the guide-shoe holes after welding. Cut them 2 mm undersize in the flat, then drill in the fixture that sets the stile spacing. Welding moves a 5 mm bracket by 0.3–0.5 mm, which is the whole positional tolerance.
- Put the adjustment in the slot, not in the hole. A 14 mm slot for an M10 bolt gives ±2 mm of movement at the shoe, which is what the installer needs for rail clearance, and it costs nothing at the laser.
The hole-to-bend distance guide owns the full rule, including how V changes with thickness, and the bending tolerance guide covers what a press brake can hold on a 2,000 mm part.
Weld sequence and the verticality that follows
A sling is welded in a fixture, and the fixture decides whether the part is plumb. Ours locates on the bottom plank, the two stile inner faces and the guide-shoe pads, with the stile spacing set by fixed stops rather than by clamps that can drift.
The sequence:
- Tack the crosshead to both stiles, checking the diagonal before any weld is deposited.
- Weld the crosshead joints on opposite sides in pairs, so the shrinkage on the left stile matches the right.
- Weld the shoe brackets last, after the frame is square, because they are the faces that get machined or drilled to final position.
- Keep the frame clamped until it is below 50 °C. A sling released warm takes a set as the last welds cool.
A 5 mm fillet on 5 mm plate is a full-thickness joint at the crosshead and needs about 15 mm of clear access for a 45° torch angle. A gusset drawn 10 mm from the stile stops the welder reaching the root, and the joint that gets skipped is always the one on the inside where nobody can see it. The fixture design guide covers location strategy; the sling-specific version is that every locating pin should sit on a face that is not subsequently welded.
Distortion in a sling shows up as a bow in the stile, and a 2 mm bow over 2,000 mm is 1 mm/m of lean, which is the whole verticality allowance. Straighten cold before coating, then re-check the diagonal and the plumb line. Never straighten after coating: the powder cracks at the yield line and the crack is a corrosion start.
Coating allowance and the guide-shoe interface
The sling is coated for the shaft environment, which is damp, dusty and often unheated. Powder at 70 µm over seven-tank pretreatment is the baseline; 80 µm is better below ground, and the pretreatment is what carries the corrosion performance rather than the extra 10 µm.
Coating changes three interfaces:
- Guide-shoe mounting faces. Two coated faces add about 0.14 mm to the joint, and because powder is compressible the shoe does not sit on a hard datum. Mask the faces, or machine a pad after coating.
- Threaded holes. An M10 thread with 70 µm on the flanks will not take a bolt. Plug every thread, and check the plugs before the booth rather than after cure.
- Slotted holes. A 14 mm slot closes to about 13.86 mm, which matters when the slot is already at the edge of its adjustment range.
Where the shoe has to be shimmed, specify the shim step on the drawing: 0.5 mm stainless shims in a stated stack, not a handful of washers. A shim stack that is not specified is a stack that gets made from whatever is in the fitter's box, and the guide shoe clearance then varies from car to car.
Inspection: four checks and one record
The inspection plan for a sling is short because most dimensions do not matter at installation. These do:
- Diagonal and squareness, with the sling standing on its bottom plank, measured across the crosshead corners and across the bottom. Equal within 2 mm.
- Stile verticality, with a plumb line or a dial gauge on a surface plate, 1 mm/m and no more than 2 mm over the full stile.
- Guide-shoe face coplanarity and spacing, measured with a straightedge and a gauge, 0.3 mm across the pair and ±1 mm on stile spacing.
- Weld visual to ISO 5817 level C, with the crosshead-to-stile joints checked for undercut and toe condition, since those are the fatigue locations.
The record is the first article dimensional report, with the coating DFT readings and the adhesion result attached. ISO 13920 gives the general tolerance classes for welded construction, and a sling drawing should name the class so the unwritten dimensions have a home. PPAP documentation is available where the lift OEM's quality system calls for it.
The Ganesh route for slings, frames and brackets
Stiles and crosshead plates are cut on the 12 kW fibre laser with the 40 ft × 16 ft bed, which takes a 2,500 mm stile and its shoe holes in one pass. Forming is on the 200 T, 5-axis brake with the 4,450 mm bed, so a crosshead channel is bent in one set-up rather than in two. Frames are welded in fixtures on the three CLOOS cells or at the manual stations, and the guide-shoe holes are drilled after welding in the same fixture that set the spacing.
Coating runs on the seven-tank line, through the 18 ft × 14 ft powder booth and the curing oven, with the masking schedule carried from the drawing. Counterweight frames, car frames and slings for TKE and Eros Elevators run on this route, and the elevator parts range covers the bracket and panel families around them.
The sling sits inside the elevator and lift components hub. The car frame manufacturing route covers the frame the sling carries, and the counterweight frame design guide covers the other half of the same suspension system.
What to put on the sling drawing
- Car size, rated load and the suspended mass, so the deflection check has an input.
- Stile centres and the datum they are measured from, with a tolerance.
- Crosshead section, span and the deflection limit, stated as a fraction of span.
- Guide shoe type and bolt pattern, with the slot dimensions and the adjustment range.
- Verticality and diagonal acceptance, with the measurement method and the standing condition.
- Weld map with leg size, continuity, ISO 5817 level and the ISO 13920 tolerance class.
- Post-weld operations: which holes are drilled after welding and which faces are machined.
- Coating system, DFT and the masking schedule, including the guide-shoe faces and every thread.
The RFQ checklist lists the volume, release and packing fields that sit alongside these, and it is worth sending with the drawing because a sling quotation without a release pattern is a guess.
When a fabricated sling is the wrong answer
Fabrication is the wrong route when the sling is a machined or cast assembly with integrated guide-shoe heads, which some high-speed and machine-room-less designs use. Those parts carry the shoe in one piece with no weld toe and no reamed bracket, and quoting them as fabrications produces a heavy part with welds in places the designer deliberately avoided.
It is also wrong when the car is large enough that the sling needs a structural calculation to a lift code and a witnessed test. We build to drawing, weld to ISO 5817 and inspect dimensions; we do not run the structural verification or the type test that EN 81-20 requires, and a sling outside the ordinary 630–1,600 kg car range should have that analysis done by the lift designer.
Aluminium slings exist and are lighter, but the stiffness drops to about a third at the same section, the weld procedure changes, and the steel guide-shoe brackets need isolation to avoid galvanic corrosion. That is a different design, not a material substitution on an existing drawing.
A drawing with the stile centres, the crosshead span, the shoe faces, the verticality callout and the masking schedule is enough for a quotation that holds. Send the current revision through the RFQ route and the section check, the fixture plan and the inspection method come back with the price.
Frequently asked questions
What is a lift sling assembly made of?
A traction lift sling is usually built from 100 × 50 × 5 mm or 120 × 60 × 5 mm RHS stiles, a formed or boxed crosshead in 6–8 mm plate, a bottom plank or underslung beam, and welded guide-shoe brackets. Material is IS 2062 E250 or an equivalent structural grade, chosen for weldability and stiffness rather than for strength.
How much deflection is acceptable in a sling crosshead?
Span/1,000 is the working limit for a car sling: 1.3 mm over a 1,300 mm span. A 120 × 80 × 6 mm box crosshead calculates to 0.81 mm at a 16 kN central load, which leaves margin for the rope hitch and any hole penetration. Beyond span/800 the guide shoes start to feel the movement.
Where do sling tolerances actually matter?
At the guide-shoe mounting faces, the stile spacing and the frame verticality. The car body hides a 2 mm error in the crosshead, but a 1 mm/m lean puts the guide shoes hard against the rail and shows up as noise and rail wear on the first run.
Should the sling be welded or bolted?
Weld the primary structure and bolt the guide shoes and the car body mounts. Welding gives the stiffness that keeps the sling square, while bolted guide shoes give the ±5 mm adjustment the installer needs to set rail clearance without shimming the whole frame.
How does coating change a sling's fit?
70 µm of powder on each face adds about 0.14 mm to a bolted joint and closes a 14 mm slot by the same amount. Mask the guide-shoe faces and any machined pad, and specify the shim step so the installer is not stacking washers to recover a millimetre.
Sources and further reading
- EN 81-20 — Safety rules for the construction and installation of lifts, covering car sling strength, guide shoe clearances and suspension · reviewed 3 Nov 2026
- IS 2062 — Hot rolled medium and high tensile structural steel, the plate and section grades used for slings · reviewed 3 Nov 2026
- ISO 13920 — General tolerances for welded constructions, dimensions for length and angle, and straightness · reviewed 3 Nov 2026
- ISO 5817 — Quality levels for imperfections in fusion-welded steel joints, the visual acceptance level for sling welds · reviewed 3 Nov 2026
Have a sling drawing to review?
Send the car size, rated load, stile centres, guide shoe type and coating specification. Our engineering team checks the section, the fixture and the weld sequence before quotation.
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