🏗️ Rigging and hydraulics

Sling angle and load per leg

With the angle measured between the sling leg and the horizontal:

Load per leg = load ÷ (number of legs × sin angle)

• The lower the angle, the higher the tension. Below 30° tension climbs fast, so avoid it unless an engineer has planned the lift.
• With 3 or 4 legs, assume only 2 legs carry the load unless the lift is engineered with equalisers.

Hitch basics

• Vertical: one leg straight up. Rated at the sling's vertical capacity.
• Choker: the sling chokes around the load. It's typically rated at about 75% of vertical (lower for some sling types and sharp choke angles), so check the tag.
• Basket: both ends on the hook. That's up to twice the vertical capacity with legs vertical, reduced by the sling angle factor as the legs spread.
• Always go by the sling's tag. Protect slings from sharp edges, and never exceed the working load limit (WLL) of any part of the rigging.

Estimating steel weight

• Carbon steel weighs about 490 lb/ft³ (0.2836 lb/in³), or 7850 kg/m³.
• A steel plate 1 in thick weighs about 40.8 lb per square foot.
• Aluminium is about one-third the weight of steel (about 169 lb/ft³, 2700 kg/m³).
• Weight = volume × density. Use the tank and area formulas on the shop math page for volume.

Hydraulic cylinder force

• Force = pressure × area. psi × in² = lbf; bar × cm² × 10 = N (1 bar acting on 1 cm² = 10 N).
• Piston area = 0.7854 × bore². Retract force uses the annulus area: 0.7854 × (bore² − rod²).
• Speed = flow ÷ area. In US units, in/min = (gpm × 231) ÷ in².
• These are theoretical figures. Real force is lower because of friction and pressure losses.