About This Guide
Towline (or towpath) systems use a floor-embedded chain or cable driven by a motor to pull carts continuously through a production facility. They're the backbone of lean production flow in automotive plants, appliance manufacturers, and large distribution centers. The casters on towline carts are not general-purpose industrial casters, they're precision components that must be engineered to the specific towline geometry, cart dimensions, and load requirements.
Selecting the wrong towline caster is costly: poor tracking, premature wear, cart derailment, and, in worst cases, production line stoppage.
Key Considerations
How Towline Systems Work
A towline system consists of the tow chain or cable (embedded in a slot in the floor, driven by a motor at a controlled speed), the tow pin (a pin on the cart that engages the chain and is pulled by it), the cart (rides on four casters guided by the chain), and pusher dogs (mechanical components on the chain that engage and disengage the tow pin at specific points).
The cart doesn't have its own motor. It's pulled by engaging with the moving chain. Casters provide rolling support and steering. The caster geometry must be precisely matched to the cart wheelbase and the towline curve radius to ensure the cart tracks the towline correctly through turns.
Critical Specification: Swivel Lead
Swivel lead is the horizontal distance from the swivel axis (centerline of the kingpin) to the center of the wheel contact point. This is the single most important dimension when specifying towline casters.
Why it matters:
- Swivel lead causes the wheel to self-align and trail behind the swivel axis when the cart is being pulled
- Insufficient swivel lead: the wheel doesn't self-align quickly enough in curves, the cart may derail or track outside the intended path
- Excessive swivel lead: the wheel over-steers in tight curves, creates side loads on the caster and makes the cart difficult to manually push when off-line
The correct swivel lead is determined by your cart's wheelbase (the distance between the front and rear caster center lines) and the minimum curve radius of your towline path. General guideline: swivel lead approximately equal to the wheel radius is a starting point for most general industrial towline applications.
Rigid vs. Swivel Caster Configuration
Configuration 1: 2 Swivel (lead end) + 2 Rigid (tow end)
The swivel casters are at the front (direction of travel) and track through curves. Rigid casters at the rear (near the tow pin) provide stability. More common in systems where the tow pin is near the rear of the cart. Swivel casters must have the engineered swivel lead, and rigid casters must be aligned precisely with the cart's longitudinal axis.
Configuration 2: 4 Rigid Casters
All four casters fixed in the direction of travel. The cart tracks the towline entirely through the tow pin connection. Simpler caster specification, less forgiving of towline geometry variations. Common in high-speed, straight-line dominant towline systems.
Configuration 3: Specialty Towline Casters
Some towline systems use proprietary caster designs with specific tow pin integration, side guides, or positive track-following mechanisms. In these cases, the caster specification comes from the system manufacturer.
Load Calculation for Towline Applications
Towline casters experience different load profiles than manually pushed carts:
- Static load, weight of cart + load, distributed per caster (standard calculation)
- Dynamic start load, when the tow dog engages, there's an acceleration force. Size accordingly.
- Curve side load, as the cart navigates a curve, centrifugal force creates side loads on the casters. Kingpinless swivel rigs handle these loads significantly better than standard kingpin designs.
Safety factor recommendation for towline applications: 1.5x minimum.
Swivel Rig Selection: Kingpinless is Standard
For towline casters carrying any significant load, kingpinless (raceway) swivel rigs are the standard, not optional. The combination of continuous dynamic loading, frequent direction changes at curves, and high cycle volumes creates conditions that destroy standard kingpin rigs quickly. Kingpinless rigs distribute load through a full ball-bearing raceway, dramatically extending service life in towline applications.
Brake Specifications: No brakes on casters used in the tow path. Brakes interfere with towline engagement and can cause carts to stop unexpectedly in the system, a serious safety and downtime risk. Separate parking/stationary brakes may be located on the cart body or tow pin mechanism (not the casters) for use at work stations.
Caster Maintenance in Towline Systems
Towline carts are high-cycle equipment. Caster maintenance matters more here than in most applications:
Daily: Visual inspection of caster wheels for flat spots or debris. Verify swivel rigs move freely (no grinding or stiffness).
Weekly: Check swivel rig torque on kingpin designs (kingpinless requires less frequent attention). Inspect wheel tread for wear patterns.
Monthly: Measure swivel lead on swivel casters, excessive wear can change the effective lead dimension and affect tracking. Replace any casters showing flat spots, cracked wheels, or bearing noise.
Annually: Consider complete caster replacement on high-cycle carts as preventive maintenance, the cost of a planned caster replacement is a fraction of an unplanned towline stoppage.
Towline Wheel Material Comparison
| Wheel Material | Best For | Rolling Resistance | Load Capacity |
|---|---|---|---|
| Polyurethane | Most towline carts (sealed concrete) | Low | Good at appropriate diameters |
| Phenolic | Heavier loads, higher-temp environments | Moderate | Higher than polyurethane at same size |
| Nylon | Coolant/chemical exposure along path | Low on clean concrete | Superior chemical resistance |
Working with Towline System Engineers
If you're installing a new towline system or upgrading an existing one, work directly with the system engineer to specify casters. The key information to provide:
- Cart wheelbase (front-to-rear caster center distance)
- Cart width (left-to-right caster center distance)
- Maximum cart + load weight
- Towline speed (feet per minute)
- Minimum curve radius in the system
- Floor type and condition
- Environmental conditions (temperature, chemicals, moisture)
Caster Central can work with your engineering team to specify towline casters to the system requirements.
Frequently Asked Questions
Swivel lead is the horizontal distance from the swivel axis to the center of the wheel contact point. It causes the wheel to self-align and trail behind the swivel axis when pulled. Insufficient lead causes cart derailment in curves; excessive lead causes over-steering and side loads. The correct lead depends on your cart's wheelbase and the minimum curve radius of your towline path.
Towline casters experience continuous dynamic loading, frequent direction changes at curves, and high cycle volumes, conditions that destroy standard kingpin rigs quickly. Kingpinless (raceway) rigs distribute load through a full ball-bearing raceway, dramatically extending service life. For any towline cart carrying significant load, kingpinless is the standard, not optional.
No. Never put brakes on casters used in the tow path. Brakes interfere with towline engagement and can cause carts to stop unexpectedly, a serious safety and downtime risk. If parking brakes are needed at work stations, use separate mechanisms on the cart body or tow pin, not on the casters themselves.
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