Reverse engineering for rings, rider rings and packing
When there is no drawing, or the drawing is wrong. We work from a worn sample and the machine data sheet to a verified manufacturing drawing you can put out to any maker.
Why it matters
The drawing is usually the problem
Most compressors in service are older than their documentation. The original maker has been bought twice, the part number no longer resolves, and the only drawing on site is a quotation sheet marked “not to scale, dimensions nominal”.
Copying that sheet reproduces its errors. Verifying it takes engineering, and engineering is what turns a dimensional copy into a part that lasts a maintenance cycle.
- No drawing exists, only a worn part
- The drawing is a quotation sheet, not a production drawing — no tolerances, no material spec, geometry drawn schematically
- The part number is obsolete or the maker has withdrawn support
- Delivery from the original maker is measured in months
- The existing part keeps failing and nobody knows why
Case study
Ø485 rider ring, first stage of a hydrogen make-up compressor
Three-stage, four-row, opposed balanced machine. 370 rpm, 320 mm stroke, 3.95 m/s mean piston speed, lubricated cylinders. The customer supplied the original maker’s quotation sheet and asked for a drawing to have the ring made. Verifying that sheet turned up four errors.
Finding 1
The cut angle contradicted the geometry. The angle table gave a straight cut at 0°, while the joint was drawn at 10°. Measured off the vector geometry, both cut faces ran at 10.0° and even the leader line carrying the 0° label sat at 10.02°.
Finding 2
The angle table was reproducible, so we could trust it. We derived the groove index relation from the ring width and cut angle, then tested it against the customer’s own drawing for the same machine: it returned the printed value to within 0.002°.
The same approach on the closed-gap check diameter matched that drawing exactly, on both the nominal and the tolerance field.
Finding 3
Two tolerance fields were two ISO grades too wide. The outer diameter carried +0/−0.40 mm where the applicable ISO 286 grade for a cut rider ring at that size is IT9, or 0.155 mm. The axial width had been carried over from a larger drawing without recalculating for the smaller nominal.
Finding 4
The outer diameter was 1.06 mm out. The sheet gave the cylinder bore, not the ring. From the groove root diameter and radial thickness the ring is 1.06 mm smaller.
Field measurements settled it independently: the calculated piston seating for the corrected diameter is 3.03 mm at the top and 1.97 at the bottom, and the service report for that machine records 2.90 and 1.95. The drawing value would have sat centred, with both clearances equal.
The material trap. The end gap on that drawing had been sized for the worst material in a broad class rather than the compound actually fitted. Compared like for like — same temperature rise, same safety factor — the published expansion figure for the fitted material calls for 10.2 mm instead of 15.8. A third less, not a third of the value.
It matters far more in the other direction. At this stage’s operating temperature the fitted 15.9 mm gap closes by 5.5 mm with the original material, leaving 10.4 mm. An equivalent quoted at 140 × 10⁻⁶/°C instead of 49.8 would close it by 15.6 mm: the gap disappears and the ring seizes. That is why we state the coefficient of thermal expansion, the test method and the direction of measurement on every material equivalence we issue — and refuse to accept a single undirected figure from a supplier.
Method
How the work runs
Five steps, typically two to three weeks from receiving the sample and the machine data.
Measure and read
The worn sample, any existing drawing, the machine data sheet and the last service report. Wear patterns often say more than the drawing.
Recover the geometry
Every derived dimension gets a formula, and every formula is validated against a drawing whose printed values are already known correct.
Verify against the standards
Bearing load to API 618, tolerance grades to ISO 286, clearances and end gap against thermal growth at the actual operating temperature.
Settle the material
Identify the original compound, propose equivalents with published data, and state what has to be confirmed before substitution.
Issue the drawing and the report
The drawing to make the part, and the calculation behind every number on it, so your inspector can check the work rather than take it on trust.
Deliverables
What you receive
Everything needed to place the order with any manufacturer, and everything needed to check our work.
- Manufacturing drawing, PDF and DXF — full dimensions, tolerances, surface finish and material
- Calculation report — every derived value with its formula and source
- Material specification with equivalents, substitution conditions, and what must be verified
- Inspection sheet for incoming goods — what to measure, at what temperature, to what limits
- Supply of the finished part, if you want it made rather than drawn
What we need from you
Send this and the work can start
If some of it is missing we will say what can and cannot be concluded without it, rather than filling the gap with an assumption. A worn sample plus the API 618 data sheet is usually enough to begin.
Essential
- The worn part, or a drawing of it
- Cylinder bore and piston outside diameter
- Groove width, depth and root diameter, with tolerances
- Reciprocating weight, or piston and rod masses
- Speed and stroke
- Suction and discharge pressure and temperature for the stage
- Gas composition, and whether liquids or particulates are present
- Lubricated or non-lubricated, and the oil grade and feed rate
Helpful
- Cylinder liner material, hardness and surface finish — a polished liner shortens ring life whatever the ring is made of
- Service life achieved with the current part, and running hours
- Measured wear of a used ring against its hours — one measurement gives a wear rate better than any catalogue figure
- Failure history and the mode observed
- Machine make, model and frame
- Expander specification, where one is fitted
Start the enquiry
Fill in what you have. Anything you leave blank we will ask about — or tell you what can’t be concluded without it. Fields marked * are required.
Send us the part
Tell us what the machine is and what failed. We will say what we can establish from what you have, what is missing, and what it will cost, before any work starts.
Figures on this page come from a single completed project and are given as an illustration of method, not as a performance claim.