10.3 Hitch Capacity Calculations

10.3 Hitch Capacity Calculations

Quick Answer: Stack the math in order: (1) identify the hitch, (2) convert vertical WLL to hitch capacity (choker ~75%, basket ~200% if legs vertical), (3) divide load by engaged legs using true CG share, (4) multiply by the angle factor from horizontal. Exam traps include forgetting the angle factor, claiming 200% basket when legs are flat, and assuming equal share when CG is off center.

This section is pure application. You already know hitch percentages (§10.1) and angle factors (§10.2). Now you combine them the way Level I written items and practical judgment demand.

Universal Workflow

1. Read the problem for load weight W, sling vertical WLL (or tag hitch WLLs), hitch type, number of legs, angles, and CG notes.
2. Establish hitch-based usable capacity of each sling/leg.
3. Find vertical share per leg (equal share only if justified).
4. Apply angle factor → leg tension T.
5. Compare T to usable capacity (and confirm crane/hook/shackle ratings separately).
6. State go / no-go and the fix if no-go.

Block A — Vertical → Choker → Basket Conversions

Problem 1 — Three hitch capacities from one vertical rating

Given: Vertical WLL = 5,000 lb. Use 75% choke and 200% vertical basket.

Checks:

• Load 4,800 lb vertical → OK.
• Load 4,800 lb choke → No (4,800 > 3,750).
• Load 4,800 lb vertical basket → OK on hitch capacity.

Problem 2 — Tag overrides the 75% rule of thumb

Given: Tag: vertical 10,000 lb, choker 7,000 lb, basket 20,000 lb. Load 7,500 lb.

Choker: 7,500 > 7,000 → reject choke even though 0.75 × 10,000 = 7,500 would have looked like a razor-edge pass under a pure 75% assumption. The printed 7,000 lb governs.

Problem 3 — Basket is not a free double if you really need a choke

Given: Vertical 6,000 lb → theoretical vertical basket 12,000 lb. Load is a slick pipe bundle that will roll out of a basket. Crew wants to “call it a basket” while actually choking.

Correct path: rate as choker → 0.75 × 6,000 = 4,500 lb. If load is 8,000 lb, neither a real choke on this sling nor a fake label is acceptable—change gear or method.

Block B — Multi-Leg With Angle Factors

Problem 4 — Equal two-leg at 60°

Given: W = 9,000 lb, two equal legs, 60° from horizontal (factor 1.155). Each leg vertical WLL = 6,000 lb.

Share = 4,500 lb.
T = 4,500 × 1.155 = 5,197.5 lb ≈ 5,200 lb.
5,200 ≤ 6,000 → OK on leg capacity.

If angle were 30° (factor 2.0): T = 4,500 × 2 = 9,000 lb > 6,000 → No-go.

Problem 5 — Equal two-leg at 45° with choker legs (stacked derates)

Given: W = 8,000 lb, two legs each choked, equal share, 45° (factor 1.414). Each sling vertical WLL = 10,000 lb; choker = 75%.

Usable per leg = 0.75 × 10,000 = 7,500 lb.
Share = 4,000 lb.
T = 4,000 × 1.414 = 5,656 lb.
5,656 ≤ 7,500 → OK.

If someone forgot the choke derate and only checked 5,656 vs 10,000, they would still pass—but if W rose to 12,000 lb:
Share 6,000; T = 6,000 × 1.414 = 8,484 lb > 7,500 choke capacity → fail, even though 8,484 < 10,000 vertical.

Problem 6 — Vertical-leg basket vs angled basket

Given: Single sling vertical WLL = 4,000 lb.

Case 6a — True vertical basket: capacity ≈ 8,000 lb. Load 7,000 lb, balanced → OK if geometry holds.

Case 6b — Basket legs at 60° from horizontal, equal share:
Do not blindly use 8,000 lb as if legs were vertical. Treat each supporting part’s tension. For a simple model with two parts sharing W equally:
Share = 3,500 lb each part; T = 3,500 × 1.155 ≈ 4,043 lb.
Each part’s vertical capacity is 4,000 lb → 4,043 > 4,000 → marginal/fail. The “200% = 8,000 lb” story assumed vertical legs.

Problem 7 — Finding maximum load for a two-leg bridle

Given: Each leg usable capacity 5,000 lb, angle 45°, factor 1.414, equal share.

W_max = (2 × 5,000) / 1.414 ≈ 7,072 lb.

At 90°, W_max = 10,000 lb.
At 30°, W_max = 5,000 lb.

Block C — Unequal CG Share

Problem 8 — Two pick points, offset CG, vertical legs

Given: W = 15,000 lb. Horizontal distances from CG: left pick 3 ft, right pick 6 ft (span 9 ft). Legs vertical (factor 1.0).

Left share (near) = W × (opposite distance) / span = 15,000 × 6 / 9 = 10,000 lb.
Right share = 15,000 × 3 / 9 = 5,000 lb.

If each leg WLL is 9,000 lb: left fails (10,000 > 9,000) even though average is 7,500 lb.

Problem 9 — Same CG offset at 60°

Use Problem 8 shares, factor 1.155:

Left T = 10,000 × 1.155 = 11,550 lb.
Right T = 5,000 × 1.155 = 5,775 lb.

Angle multiplies the already unequal shares. Off-center + low angle is a double penalty.

Problem 10 — Four-leg “hope” vs two-leg reality

Given: Four-leg bridle on a rigid load with high CG; only two legs will engage until the load tips. W = 20,000 lb. Someone divides by four: 5,000 lb/leg—wrong.

If only two legs work: share 10,000 lb each before angles. At 45°, T ≈ 10,000 × 1.414 = 14,140 lb per engaged leg. Level I takeaway: only count legs that share load; do not assume four-leg capacity without true engagement.

Exam Traps — Explicit Checklist

Additional Worked Mini-Set (Speed Practice)

M1. Vertical 12,000 lb sling, choke 75%: choke cap = 9,000 lb.
M2. W = 6,000 lb, two equal legs @ 90°: T = 3,000 lb.
M3. W = 6,000 lb, two equal legs @ 30°: T = 6,000 lb.
M4. Vertical 3,000 lb → vertical basket ≈ 6,000 lb.
M5. Tag choke 4,800 lb, load 5,000 lb choked → No-go.
M6. Shares 7,000 and 3,000 lb at 45°: T ≈ 9,898 lb and 4,242 lb (×1.414).
M7. Each leg 8,000 lb usable, 60°, equal: W_max ≈ 16,000 / 1.155 ≈ 13,853 lb.
M8. 0.80 × 5,000 vertical = 4,000 lb choke if tag uses 80%.

Decision Language for “No-Go” Answers

When capacity fails, exam-correct responses sound like:

• Stop and change hitch, add or resize slings, or revise pick points per the lift plan.
• Do not proceed slowly to “test” an overloaded choke.
• Do not rely on crane spare capacity to fix sling overload.
• Re-measure angles if the factor was based on a guessed angle near a limit.

Section Synthesis

Capacity math is mechanical if you refuse to skip steps. Hitch percentage without angle is incomplete; angle without hitch type is incomplete; equal-share assumptions without CG are incomplete. Work Problems 1–10 until the trap list feels obvious. Edge protection and bend ratios in §10.4 can still remove capacity even when the pure arithmetic passes—geometry of the contact surface is the last execution filter.