PROJECT DESIGN REVIEW — MINUTES OF MEETING
Project: rotary turntable station for paint pre-treatment, 1.8 m platen
Review subject: slewing bearing size, race type and mounting bolt pattern
Date: 18 June, 09:30 to 11:00, meeting room 2
Document ref: DMR-2026-0618-TRT05
Attendees
| Name | Role |
|---|---|
| R. Fontaine | Lead structure designer (chair) |
| I. Marchenko | Workshop fitter, turntable assembly |
| E. Duval | Loads and fatigue engineer |
| N. Bakker | Quality / inspection |
| S. Rossi | Procurement |
1. Purpose of the review
The turntable carries a 920 kg platen with four paint fixtures, rotating at 6 rpm for the pre-treatment spray. The draft used a single-row ball slewing bearing of 760 mm outside diameter, with a rated dynamic load of 245 kN. During the design check the table was found to be sitting on the bearing selection table as if any row size would do, and the review was called to put a method behind the choice before the floor plates were drilled.
2. Review comment TR-01: the axial, radial and moment loads in one vector
E. Duval opened with the load state, and the first slide was a diagram the design office had never drawn: the bearing does not see the platen weight alone. The loads on the slewing bearing are the axial weight, a radial force from the spray reaction and the unbalanced fixture mass, and a tilting moment from the center of gravity sitting off the rotation axis. The measured numbers were written together: axial load 12.4 kN from platen, fixtures and plated parts at 1.1 g, radial load 3.1 kN from the spray nozzle reaction and conveyor drag, and a tilting moment of 7.8 kN-m because the heaviest fixture sits 320 mm off the axis with a 3.4 kN part load during the spray. E. Duval stressed that the tilting moment is the load that kills a single-row ball bearing, because it pushes the upper race to a line contact while the lower race unloads, and the bearing catalogues express their capacity on these three components together, not on the axial load alone.
3. Review comment TR-02: single-row ball versus crossed roller versus triple row
R. Fontaine laid out the three families side by side on the whiteboard. A single-row ball slewing bearing carries the moment through the balls in a four-point contact, cheap and compact, best where the moment is small. A crossed roller bearing uses cylindrical rollers set radially and axially in a single race, with a higher moment capacity for the same envelope because the rollers take the tilt in rolling contact. A triple-row roller bearing splits the axial and radial rows and carries the largest moments, but it is the deepest and the most expensive. For the 7.8 kN-m moment and the 12.4 kN axial load, the single-row ball bearing had a calculated capacity utilization of 1.08, meaning it was asked to do eight percent more than its rated combination, and was rejected before the design office could argue.
4. Review comment TR-03: the necessary capacity check that failed the first candidate
E. Duval walked the meeting through the catalogue check, which compares a utilization factor. The catalogue gives, for each bearing size, a rated moment capacity at a given axial load, and the check is that the actual moment divided by the rated moment stays under 1.0 once the service factor is applied. For the 760 mm single-row ball, the rated moment at 12.4 kN axial was 7.2 kN-m, and the applied moment of 7.8 kN-m times a duty factor of 1.15 gave 8.97 kN-m, a utilization of 1.25. The meeting noted the numbers stayed on the board and moved to the next candidate rather than argue with arithmetic.
5. Review comment TR-04: the crossed roller candidate in the same envelope
The review then checked a crossed roller bearing in the same 760 mm class. The catalogue rated its moment capacity at 16.5 kN-m at the same 12.4 kN axial load, more than double the single-row ball for the same bolt circle and race height. With the duty factor applied, the utilization dropped to 0.54, which slides a bearing half-loaded into a machine that should be over the rated line at a margin the customer will accept. I. Marchenko asked what the price of that margin was, and S. Rossi gave the procurement figure: the single-row ball was 1,180 euros, the crossed roller 1,690 euros, a 510 euro difference on a station the plant quotes at 48,000 euros. The discussion noted that the 510 euros buys more than margin; it buys a bearing that will not need its race reground when the platen picks up a heavier fixture in two years.
6. Review comment TR-05: race hardness and the hardness of the mounting rings
N. Bakker brought a subject the minutes of earlier reviews rarely captured, the hardness chain from bearing race to machine structure. The slewing bearing is hardened to 58 to 62 HRC on the raceway, but it bolts onto a mounting ring of structural steel at 180 HB, and under a moment load the soft ring deflects away from the race, unloading the bolts on the tension side and overloading the race on the compression side. N. Bakker’s rule, learned from a bearing that failed at 90,000 cycles in an earlier machine, was to hold the mounting ring faces to a flatness of 0.05 mm and to machine both the turntable and the chassis ring in one setup so the deflection under load stays inside the catalogue assumption. The original drawing called for a commercial tolerance of 0.2 mm flatness, which the review accepted only as the first-pass figure, with the machining change written as an action item.
7. Review comment TR-06: the bolt pattern is part of the bearing, not a detail
R. Fontaine asked the workshop how the bearing had been bolted on the previous turntable, and I. Marchenko’s answer produced the longest silence of the meeting: the previous machine used the same bolts with the same torque, twenty M12 grade 8.8 bolts at 95 N-m, and it worked, so nobody had checked whether it was right. The review checked: the catalogue gives a maximum bolt load for the bearing ring, and for the crossed roller the twenty 8.8 bolts at 95 N-m pull only 68 percent of the catalogue clamping requirement once the moment is added. The fix that kept the bolt count was to move to grade 10.9 bolts at 130 N-m, which raises the clamp above 100 percent and fits the existing twenty holes. The minutes noted that a bearing is only as stiff as the bolts that hold the ring in the direction of the race.
8. Disagreement: crossed roller against the single-row ball retaining the axial float
The debated point was not the bearing family but the axial float. The turntable passes a pre-treatment spray and a heating zone, and the platen sees a temperature swing that nobody had quantified. E. Duval argued the crossed roller bearing, being axially preloaded by its roller geometry, would lock the platen and translate the thermal growth into a race load, while the single-row ball with a clearance fit would let the platen breathe. I. Marchenko answered from the floor that the thermal growth had been measured on the old table at 0.12 mm over a two-hour shift cycle, and that the new design could place a floating connection at the platen-to-shaft interface instead of the bearing, keeping the crossed roller fully constrained at the race and letting the growth exit at the top of the column. That answer removed the objection, and the meeting agreed the float belongs at the coupling, not at the bearing.
9. Risks and mitigation
Three risks were logged beside the decision. Risk 1: the crossed roller race is sensitive to mounting ring flatness, and a distorted ring would show as a hard spot in rotation within days; mitigation is the single-setup machining at 0.05 mm flatness with an inspection report. Risk 2: the grade 10.9 bolts raise the clamp but also raise the risk of galled threads; mitigation is a light oil on the threads and a stretch check on the first six bolts. Risk 3: the heavier fixture in two years could push the 0.54 utilization toward the rated line; mitigation is to log the actual fixture weights per quarter so the margin is re-checked against the real duty.
10. Actions and owners
| ID | Action | Owner | Deadline |
|---|---|---|---|
| ACT-1 | Order crossed roller slewing bearing 760 mm class | S. Rossi | 22 June |
| ACT-2 | Machine turntable and chassis rings in one setup, 0.05 mm flatness | I. Marchenko | 5 July |
| ACT-3 | Change ring bolts to grade 10.9, torque 130 N-m | I. Marchenko | 8 July |
| ACT-4 | Fit floating coupling at platen to shaft interface | R. Fontaine | 12 July |
| ACT-5 | Record running torque at first rotation, log quarterly fixture weights | N. Bakker | from 15 July |
11. Decision summary
Adopt the crossed roller slewing bearing in the 760 mm class for the 1.8 m turntable. Confirm the moment utilization at 0.54 against the 7.8 kN-m tilting moment and 12.4 kN axial load. Machine the mounting rings flat to 0.05 mm in one setup. Bolt the ring with grade 10.9 fasteners at 130 N-m. Carry the float at the platen-to-shaft coupling so the bearing stays fully constrained.
12. Post-meeting addendum, 30 July
The turntable indexed its first full shift with a hand-held torque reading of 58 N-m for the crossed roller against 71 N-m for the old single-row ball at the same speed, a drop that surprised nobody who had heard the acceptances. The flatness report came back at 0.04 mm, under the 0.05 mm target, and the twenty grade 10.9 bolts all checked within five percent of the 130 N-m stretch estimate. The plant has asked for the same bearing family on the two smaller stations next year.
Glossary
Slewing bearing: a large-diameter ring bearing used for slow, partial or full rotation. Crossed roller bearing: rollers laid alternately at right angles in a single race, carrying axial, radial and moment loads. Utilization factor: applied load combination divided by rated capacity, held below 1.0. Axial float: permitted length change along the rotation axis, often needed where thermal growth exists.
Appendix. The sizing worksheet the meeting signed off
The worksheet froze the numbers so a future design can repeat the path. Duty: 920 kg platen and fixtures, 6 rpm index, axial 12.4 kN, radial 3.1 kN, tilting moment 7.8 kN-m, duty factor 1.15. Candidate A, single-row ball 760 mm, rated moment 7.2 kN-m at rated axial, applied 8.97 kN-m, utilization 1.25, price 1,180 euros, rejected. Candidate B, crossed roller 760 mm, rated moment 16.5 kN-m at the same axial, applied 8.97 kN-m, utilization 0.54, price 1,690 euros, adopted. Candidate C, triple-row roller 820 mm, rated moment 28 kN-m, utilization 0.32, price 3,240 euros, rejected on cost and depth because the 0.54 utilization of B was already inside the comfort band the plant likes between 0.4 and 0.7.
The chair closed by reading the one-sentence lesson agreed by the room: a slewing bearing is selected on the tilting moment it must carry over the cycle, on the flatness of the surfaces it sits on, and on the bolts that keep it in its seat, and a bearing that passes two of the three will fail the third within a year. The minutes were signed by all five attendees, and the drawing office was tasked to update the turntable assembly so the grade 10.9 bolt note appears in the erection drawing, not only in the minutes.
