Overhead Crane End Truck Design: Wheel Loads, Axle Sizing & CMAA Compliance
Introduction
The end truck is the component that carries the entire crane. It sits at each end of the bridge, holds the wheels, and transfers every kilogram of the crane and its load into the runway rail below. Many people picture it as a simple steel box with wheels bolted on.
It is far more than that.
An end truck determines how load spreads across your runway, how long your wheels and bearings last, and whether your crane tracks straight or fights the rail every time it moves. Get the wheel load distribution wrong, and you accelerate rail wear, shorten bearing life, and add unplanned downtime that eats directly into your equipment budget.
This guide gives procurement decision-makers a clear framework for evaluating end truck design. You will learn the two main construction types, how to calculate wheel loads, how axles and bearings are sized, what CMAA duty class actually means for your operation, and the design errors that quietly drive up your total cost of ownership. Let’s start with the basics.
Part 1: What an End Truck Does and the Two Main Types
An end truck (sometimes called an end carriage or end tie) is the transverse structural member at each end of a bridge crane. It houses the travel wheels, supports the bridge girder, and carries the drive components that move the crane along the runway.
Rigid (Welded) End Trucks
A rigid end truck is a single welded steel box section. The wheels mount in fixed bearing housings, and the whole assembly bolts directly to the bridge girder.
- Best for: shorter spans and lighter duty classes.
- Advantage: lower cost, simple fabrication, easy to inspect.
- Limitation: less tolerant of runway misalignment, which can concentrate load on one wheel.
Articulated (Pivoting) End Trucks
An articulated end truck uses a pivot connection between the wheel assembly and the girder connection point. This lets the wheels adjust slightly to runway irregularities.
- Best for: longer spans, heavier loads, and higher duty classes.
- Advantage: better load equalization across all wheels, reduced flange wear.
- Limitation: higher cost and more components to maintain.
A quick way to think about this: rigid trucks reward you with lower upfront cost, while articulated trucks reward you with lower long-term wear cost. Your duty class and runway quality decide which pays off.
Part 2: Wheel Load Distribution
Wheel load distribution is the single most important input in end truck design. It governs wheel size, bearing rating, axle diameter, and the runway beam beneath.
The Maximum Wheel Load Formula
The maximum wheel load is the highest force a single wheel transmits to the rail. Under CMAA Specification No. 70:
Maximum wheel load = (Crane dead weight share + Rated load + Hoist/trolley weight) × Dynamic impact factor ÷ Number of wheels per end truck
Dynamic Impact Factors (CMAA No. 70)
The dynamic factor accounts for the extra force created when a loaded crane accelerates, stops, and vibrates during travel.
- Class A–B (infrequent use): 1.10
- Class C–D (moderate to heavy production): 1.15 to 1.20
- Class E–F (severe, continuous duty): 1.20 to 1.25
Worst-Case Trolley Position
The heaviest wheel load does not occur when the trolley sits at the bridge center. It occurs when the trolley moves to its closest approach to one end truck.
For a bridge span L with the trolley at distance d from End Truck A:
End Truck A reaction = (Bridge dead weight ÷ 2) + (Rated load + hoist weight) × (L − d) ÷ L
At minimum approach distance (typically 0.5 to 1.0 metre due to clearance), one end truck can carry 65 to 80% of the combined load and hoist weight. Divide that reaction by the wheels per end truck (2 for standard cranes, 4 for heavy cranes) to get the design wheel load.
Why this matters to you: an underestimated wheel load means undersized wheels and premature rail damage. An overestimated one means paying for capacity you never use.
Part 3: Wheel Selection and Rail Contact Stress
Once you have the maximum wheel load, you size the wheels to control contact stress — the pressure at the point where wheel meets rail.
Wheel Diameter Guidelines
Wheel diameter scales with wheel load. As a reference for double-flanged forged steel wheels:
| Maximum Wheel Load | Typical Wheel Diameter |
|---|---|
| Up to 5 tonnes | 200–250 mm |
| 5–12 tonnes | 250–400 mm |
| 12–25 tonnes | 400–600 mm |
| Above 25 tonnes | 600 mm and larger |
Contact Stress Check
Contact stress rises as wheel load increases and falls as wheel diameter and rail width increase. A larger wheel on a wider rail spreads the load and extends both wheel and rail life.
- Wheel material: forged steel, typically hardened to 300–350 HB on the tread.
- Design goal: keep contact stress within the allowable limit for the wheel and rail hardness combination.
Higher duty classes push toward larger, harder wheels because they see far more travel cycles. Enhance equipment uptime by specifying wheels sized for your real cycle count, not just your peak load.
Part 4: Axle and Bearing Sizing
The axle carries the wheel load into the end truck structure, and the bearings let the wheel rotate under that load. Both are fatigue-critical components.
Axle Sizing
The axle is sized for bending and shear from the wheel load, with a fatigue allowance for repeated rotation.
Bending moment on axle = Maximum wheel load × Distance from wheel centerline to bearing support
- Axles use medium-carbon or alloy steel (for example 42CrMo) for fatigue strength.
- Keep the working stress well below the fatigue endurance limit for the duty class.
- Higher duty classes demand a larger safety margin against fatigue.
Bearing Selection
Bearings are chosen from their dynamic load rating and required service life, expressed as L10 life — the hours 90% of bearings will reach before fatigue failure.
Basic L10 life (hours) = (Dynamic load rating ÷ Equivalent bearing load)³ × (10⁶ ÷ (60 × wheel rpm))
- Class A–C: target 10,000–20,000 hours L10 life.
- Class D–E: target 20,000–40,000 hours.
- Class F (continuous): target 40,000+ hours.
Sealed tapered roller or spherical roller bearings suit crane wheels because they carry both radial (vertical) and axial (side thrust) loads. A reliable bearing specification directly reduces your unplanned maintenance spend.
Part 5: CMAA Duty Classification (Class A–F)
CMAA duty class is the language that connects your operation to the correct end truck design. It defines how hard your crane works, and every component margin flows from it.
The Six Classes
- Class A — Standby/Infrequent: rare use, light loads (power stations, maintenance).
- Class B — Light: slow speeds, occasional loads (repair shops, light assembly).
- Class C — Moderate: regular use near rated capacity (machine shops, paper mills).
- Class D — Heavy: high-cycle production, frequent full loads (foundries, fabrication).
- Class E — Severe: near-continuous operation at or near capacity (bulk handling, magnet duty).
- Class F — Continuous Severe: 24/7 critical duty (steel mills, high-throughput plants).
Load Cycle Ranges
Duty class corresponds to expected load cycles over the crane’s design life:
| Class | Approximate Load Cycles |
|---|---|
| A | Under 20,000 |
| B–C | 20,000–500,000 |
| D | 500,000–2,000,000 |
| E–F | Over 2,000,000 |
The procurement takeaway: specifying a class below your true usage saves money today and costs far more in early wheel, axle, and bearing failure. Specifying too high wastes budget. Match the class to your measured cycle count for the best cost-reliability balance.
Part 6: Drive Configuration and Tracking
How the end truck is driven affects tracking accuracy, wear, and energy use.
Common Drive Types
- A4/A6 (individual wheel drive): each driven wheel has its own gearmotor. Best tracking, lowest skew, common on higher duty classes.
- A1 (single central drive): one motor drives both wheels through a line shaft. Lower cost, suited to lighter duty.
Why Tracking Matters
Poor tracking causes skewing — the crane travels slightly crooked, forcing wheel flanges against the rail. This accelerates flange wear, raises energy use, and can trigger rail damage.
- Variable Frequency Drive (VFD) control smooths acceleration and reduces skewing forces.
- Correct wheel alignment (parallel axles, matched wheel diameters) keeps tracking true.
Streamlined travel control extends the service interval on your wheels and reduces power draw — two measurable operating savings.
Part 7: Common Design Errors
These errors quietly increase total cost of ownership. Recognizing them helps you ask better questions before you buy.
Error 1: Sizing Wheels for Static Load Only
The wheel is chosen for the static wheel load, but the dynamic impact factor is left out. The wheels and bearings then run overloaded on every travel cycle, failing years earlier than planned.
Error 2: Ignoring Worst-Case Trolley Position
Designers use the average wheel load instead of the load at closest trolley approach. One end truck’s wheels carry far more than assumed, wearing unevenly and creating tracking problems.
Error 3: Duty Class Mismatch
A crane rated Class C is placed into Class E service. The end truck components meet the “on paper” rating but see far more cycles than they were designed for, driving early fatigue failure.
Error 4: Undersized Bearings for Side Thrust
Bearings are rated for vertical load only, with no allowance for the axial side-thrust force from acceleration and skewing. The bearings develop premature axial wear and fail unpredictably.
Frequently Asked Questions
Q: How do I know which CMAA class my operation needs?
A: Estimate your daily lifts, average load as a percentage of capacity, and hours of daily use, then map that to the load cycle ranges in Part 5. If your crane runs multiple shifts near full capacity, you are likely in Class D or above. When in doubt, ask your supplier for a duty-class assessment based on your actual usage data — it protects you from both under-buying and over-buying.
Q: Can I upgrade end trucks on an existing crane to increase capacity or duty class?
A: Sometimes, but only after a full assessment. Increasing capacity raises wheel loads, which affects wheels, axles, bearings, and the runway beam below. An upgrade often requires larger wheels, higher-rated bearings, and a check that your existing runway can handle the new loads. Treat it as a system change, not a bolt-on swap.
Q: What single factor most affects end truck service life?
A: Matching the design to your real duty cycle. Correct wheel load calculation and the right CMAA class together drive wheel, axle, and bearing life more than any other decision. Accurate inputs at the design stage deliver the biggest gains in equipment uptime and the lowest lifetime cost.
Q: How long should quality end trucks last?
A: With correct duty-class sizing and routine maintenance, wheels typically last 5–10 years and bearings meet their L10 target hours. Poorly matched designs can halve those figures, so specification accuracy pays back directly in reduced replacement spend.
Conclusion
End truck design decides how your crane spreads its load, how straight it tracks, and how long its wheels, axles, and bearings survive. The controlling factors are consistent: an accurate maximum wheel load, wheels sized for contact stress, axles and bearings rated for fatigue, and a CMAA duty class matched to your real operation.
Get the wheel load calculation right. Match the duty class to your true cycle count. Specify bearings for both vertical and side-thrust loads. Do this, and you gain measurable equipment uptime, lower maintenance spend, and a crane that runs reliably for its full service life.
Ready to specify end trucks matched to your exact duty cycle? Contact our engineering team for a free end truck design review and a cost-reliability assessment for your facility’s crane system.