The Ball Table That Wouldn’t Turn the Box
We installed a ball transfer table (a bed of ball transfer units) for manually moving boxes. The balls were small (Ø12 mm) and spaced 150 mm apart. On the bench, boxes slid easily. On the floor, a heavy box (30 kg) wouldn’t turn — it dragged between the balls. The problem: the balls were too small (low load capacity per ball) and the spacing was too wide. The box sagged between the balls and dragged on the table surface. We upsized to Ø25 mm balls spaced 100 mm apart. The box rolled. The mistake was under-sizing the ball units and spacing them too far.
Ball transfer unit design for manual handling tables is about ball size, spacing, and load capacity. This article covers the selection.
What a Ball Transfer Table Does
A ball transfer unit (BTU) is a ball (steel ball) seated in a housing. The ball protrudes above the surface. Objects (boxes, pallets) roll on the balls in any direction. Used for manual handling (loading, rotating, orienting parts) on assembly stations.
The table is a flat surface with a grid of ball units. The operator pushes the box; the balls let it move freely in X, Y, and rotation.
Step 1: Load Per Ball
The table’s load is distributed across the balls under the box. Each ball carries a fraction of the load.
Load_per_ball = box_weight / n_balls_under_box
For a 30 kg box (300 N) on a table with balls at 100 mm spacing, and the box footprint is 400×400 mm: the box covers about 4×4 = 16 balls. Load per ball = 300 / 16 = 18.75 N per ball.
A Ø12 mm ball unit is rated for about 20–30 N. That’s borderline. A Ø25 mm ball is rated for 100–200 N. Comfortable. The mistake: we used Ø12 mm balls (20 N rating) for a 30 kg box — the load per ball (18.75 N) was near the limit. The balls sank under the load, and the box dragged.
| Ball Size | Load Per Ball | Typical Spacing | Best For |
|---|---|---|---|
| Ø10 mm | 10–20 N | 75 mm | Light boxes, small parts |
| Ø15 mm | 30–50 N | 100 mm | Medium boxes (10–20 kg) |
| Ø25 mm | 100–200 N | 100–150 mm | Heavy boxes (30–100 kg) |
| Ø30 mm | 200–400 N | 150 mm | Pallets, heavy loads |
Step 2: Ball Spacing
The spacing between balls determines how well the box rolls. Too far apart, and the box sags between balls (drags). Too close, and you waste balls (cost).
Rule: the spacing should be less than half the smallest dimension of the box. If the box is 200 mm wide, the spacing should be under 100 mm. The box always sits on at least 3 balls.
For a 400×400 mm box, 100 mm spacing works (the box covers 16 balls). For a 200×200 mm box, 75 mm spacing (the box covers 9 balls). For small parts (under 100 mm), use 50 mm spacing — or a different material (a low-friction plastic sheet, not balls).
The ball table rule: Size the ball unit for the load per ball (with margin). Space the balls so the box always sits on at least 3 balls. The table that dragged the heavy box had Ø12 mm balls at 150 mm spacing — the load per ball was at the limit and the spacing was too wide. Upsize to Ø25 mm at 100 mm spacing.
Ball Material: Steel vs. Nylon
Steel Ball
A hardened steel ball. Strong, durable. But it can scratch soft parts (aluminum, plastic) and is noisy. Standard for boxes and metal parts.
Nylon (Plastic) Ball
A plastic (nylon or acetal) ball. Won’t scratch soft parts. Quieter. But lower load capacity (about half of steel). For delicate parts (painted surfaces, plastic housings).
| Ball Material | Load Capacity | Surface Safe? | Best For |
|---|---|---|---|
| Hardened steel | Full rating | No (can scratch) | Boxes, metal parts |
| Nylon/acetal | 50–70% of steel | Yes (non-marring) | Painted, plastic, soft parts |
| Stainless steel | Full rating | For washdown | Food, washdown, corrosive |
Recirculating vs. Static
Some ball units are “recirculating” — the ball rolls on a bed of smaller balls inside the housing (like a caster). Lower friction. Others are “static” — the ball sits on a single support. Higher friction but cheaper.
For heavy loads, use recirculating (low friction). For light loads, static is fine.
Table Surface Around the Balls
The balls protrude above the table surface by about 3–5 mm. The box rolls on the balls, not on the table. But if the ball height is inconsistent (some balls protrude more than others), the box rocks.
- Adjustable ball units: Some BTUs have a height adjustment (a screw from below). Set all balls to the same protrusion.
- Flat table surface: The table (aluminum plate or steel sheet) must be flat. Warped table = some balls don’t touch.
Active vs. Driven Ball Tables
A passive ball table (just balls) needs the operator to push. For heavy boxes, add powered balls (motor-driven rollers or balls). A “powered ball table” has some motor-driven balls that move the box automatically.
For manual stations, passive balls are fine. For automatic transfer between machines, use a powered roller conveyor (not balls).
A Ball Table Checklist
- What is the heaviest box/part? (kg)
- What is the box footprint? (mm × mm)
- How many balls under the box? (Grid.)
- Load per ball? (kg / n_balls.)
- Pick ball size: load per ball < ball rating (with margin).
- Spacing: less than half the smallest box dimension?
- Ball material? (Steel, nylon, stainless?)
- Recirculating or static? (Heavy loads = recirculating.)
- Are ball heights adjustable? (Consistent protrusion?)
- Is the table flat? (Warped = rocking.)
- Is it manual or powered? (Heavy boxes = powered.)
- Is there a stop/lock? (Hold the box at the work position?)
The Bottom Line
Ball transfer table design is load per ball and ball spacing. The table that dragged the heavy box had small balls spaced too wide. Calculate the load per ball (box weight / balls under box), pick a ball size that exceeds that with margin, and space the balls so the box always sits on at least three. Use nylon balls for soft parts. The table where boxes roll freely wasn’t the biggest balls — it was sized for the box weight and footprint.
