When an air bearing stage does not perform to expectation, the cause is usually not the bearing - it is an undefined requirement between the air bearing designer and the granite component manufacturer. What flatness does this pad need, over which zone? What straightness, over which travel? Who supplies the stage, and who is responsible when the assembly is measured? This article gives motion platform engineers a structured way to define granite component requirements, so that the granite supplier and the air bearing system designer can work from the same page.
1. The Role of Granite Components in an Air Bearing System
An air bearing motion system has three distinct layers, and it is essential to know which one you are buying:
- The granite component - rail, base or plate: the lapped reference geometry (our scope)
- The air bearing elements - pads, cartridges, preload and air circuit (supplied by the bearing partner or integrated as an assembly)
- The complete stage - carriage, drive, controls and calibrated system performance
The schematic below shows the division of surfaces and the parameters that must be agreed for each zone:
2. Guide Surface, Bearing Zone and Stage: Different Requirements
Not every surface of a granite rail needs lapping. A well-defined drawing separates:
- Guide zones where bearing pads run: these need flatness and roughness matched to the bearing design - an orifice pad, a porous pad and a thrust washer each impose different local requirements
- Structural zones carrying inserts and mounting datums: machined tolerances, usually far more economical
- Assembly interfaces: pad preload surfaces, limit stop faces and clean-out grooves, agreed with the air bearing partner
Because of this, a single universal flatness or roughness number is never "the standard answer" - the values follow from the bearing concept, the film thickness and the accuracy target of the stage.
3. Geometry and Surface Parameters to Confirm When Buying
For the granite component itself, confirm these items in the drawing review:
- Flatness per guide zone, with the measurement support condition (design support points)
- Straightness of each guide zone over the full travel, in both vertical and horizontal planes
- Surface roughness of lapped zones - matched to the bearing supplier's recommendation, not to a generic value
- Relative position of opposite guide zones (height difference, parallelism) for preloaded pad arrangements
- Edge chamfers, relief grooves and runway ends, which affect pad behavior at the ends of travel
4. Input Conditions the Supplier Needs From You
To propose the right specification, we need your system's inputs: load per pad and total moving mass, direction of motion and effective travel, the type of bearing element planned, air supply conditions, and the working environment (temperature stability, vibration, cleanliness). With these, the granite geometry can be engineered around your bearing scheme instead of guessed.
5. Seams, Holes, Edges and Assembly Interfaces
Multi-rail systems must state whether rails are matched and lapped as a set, and how seams are handled when travel exceeds a single block. Inserts and mounting holes follow the same rules as any precision granite part - bonded steel inserts with defined proof loads - but positions near guide zones deserve extra care, since local stress can influence the lapped surface. Assembly preload methods (tension rails versus opposed pads) change the contact geometry and should be communicated before lapping starts, not after.
6. Inspection Methods and a Sample Delivery Report
Guide zone flatness and straightness are verified with electronic levels, autocollimators or laser interferometers, always in the design support condition. The report below shows the format we deliver with every air bearing granite component - deviation map, method, ambient conditions and pass/fail against the agreed specification:
Drawing Review Checklist for Air Bearing Granite Components
- Guide zones marked and dimensioned, with flatness, roughness and straightness per zone
- Relative position of opposing rails defined (for preloaded designs)
- Inserts and holes specified with thread, depth, load and isolation requirements
- Support points defined and used as the measurement condition
- Travel, direction, end-of-travel behavior and seam handling agreed
- Bearing element type and preload method communicated by the air bearing partner
Unparalleled Ltd supplies granite components for granite air bearing stage builders and, for ceramic bearing elements, complements our range with precision ceramic air bearing products. Whether you need a single lapped rail or matched multi-rail sets with certified reports, send us your travel, load and existing bearing concept - our engineers will review the drawing and confirm the manufacturable specification with you.