What You Will Learn
- How to write a weight-tolerance method on the PI, not a slogan
- Why cast iron cores and rubber coats need separate checks
- Scale setup, sample versus bulk, and kg versus lb marking
- What this check does not prove (diameter, hole fit, drop rating)
Table of Contents
What weight tolerance means on plates
Weight tolerance on a plate is the allowed difference between the marked rating and the mass you measure on a stated scale, after a stated preparation. It is not “feels heavy.” Importers control it by putting the method on the proforma invoice: unit (kg or lb), dry or as-packed, percent of rated weight or a gram band, and whether you judge each plate or a pair.
Yunlingyu exports cast iron plate weights and rubber-coated constructions from the same buyer workflow. Cast iron is mostly the casting. A rubber-coated weight plate is a core plus a coat whose mass must be in the quote, not guessed after molding.
Where claims start
A golden sample can sit inside the band while bulk sits outside it. Rubber hides a light core. A 10 kg mark and a 22 lb mark are not the same SKU. A warehouse scale that was never checked against a known mass will create false fails and false passes. Floor claims also mix two issues: the plate is light, or the left and right plates in a set do not match. Write both rules if both matter to the gym.
Do not treat a catalog line such as “±2%” as a law. Percent of what, weighed how, at which sample size? If the factory cannot answer those three questions, you do not have a tolerance. You have marketing copy.
Core, coat, and how factories hold mass
Ask the factory to state the control point. For bare iron, the control point is the finished casting after machining the hole and any paint. For rubber-coated plates, decide whether you accept the finished plate only, or also a core-weight check before coating. Core-only control without a finished-plate check lets the coat run heavy or light.
| Construction | What actually holds the mass | Write on the PI | Typical buyer fail |
|---|---|---|---|
| Bare cast iron | Mold fill, machining, paint film | Rated kg or lb; percent or gram band; dry, no carton | Light after machining; heavy paint on small plates |
| Rubber-coated iron | Core plus coat | Finished-plate mass; optional core mass before coat | Light core hidden by a thick coat |
| Bumper (rubber-dominant) | Insert plus rubber volume | Finished mass; diameter is a separate spec | Diameter in spec, mass out of band, or the reverse |
Percent bands get wider in grams as the rating goes up. A gram band (for example “within X g of marked weight”) is clearer on small plates. Pick one method per PO. Mixing percent on bumpers and grams on 1.25 kg change plates without writing it down is how two inspectors disagree at receiving.
Weighing steps buyers can repeat
- Confirm the SKU: marked kg or lb, color, and hole type. Photograph the mark next to the scale display.
- Use a scale that can resolve the band you wrote. If the band is 50 g, a 100 g resolution scale cannot judge it. Ask the factory for the scale model and last check against a known mass.
- Weigh dry, out of the carton, with no extra bag unless the PI says “as packed.” Moisture and packing add mass that is not the plate.
- Zero the scale, place the plate centered, record the reading. Repeat once. If the two readings differ by more than the scale resolution, stop and check the scale.
- Compare to the PI method. Percent: (measured − marked) / marked. Gram band: measured minus marked. Do not switch methods on the warehouse floor.
- Sample versus bulk: weigh the approved sample with the same scale setup you will use on AQL pieces. If bulk uses a different scale, you are not repeating the test.
- Set matching: if the gym loads two plates that should match, write a pair delta as well as a single-plate band. A pair can both be inside a wide percent and still feel uneven on the bar.
kg versus lb: if the destination market is pounds, order pound-marked plates or write the conversion and rounding on the PI. Do not convert 20 kg to 45 lb on a sticker after the fact and then weigh against 45 lb. That is a marking change, not a weighing method.
What this check is not for
Mass check does not prove Olympic sleeve fit, 450 mm bumper diameter, coating adhesion, or drop rating. A plate can be inside the weight band and still have a tight hole or a crushed carton. Those are separate incoming lines. This page is not a calibration certificate and not a named laboratory standard. If your retailer requires a named test, attach that method to the RFQ. Do not print a standard mark the goods do not have.
FAQ
Is ±2% enough for commercial gyms?
Only if you define the denominator (marked weight), the scale, and whether you also need a pair delta. Many gyms care more about matched pairs than about a wide percent on a single plate. Write both if both will generate a claim.
Should rubber-coated plates be weighed before or after coating?
Weigh the finished plate against the rating the gym will see. A core check is extra process control. It does not replace the finished-plate reading.
What if the sample passes and 10% of bulk fails?
That is a process issue, not a “scale difference.” Hold the lot, share the AQL sheet and photos of the scale display, and ask for root cause: mold fill, coat thickness, or a mix of kg and lb blanks.
Do we need a third-party lab?
Not for a routine PI method. You need a repeatable scale, photos, and a written band. Use a lab only if a contract or retailer names one.
Conclusion
Control plate mass by writing the method, the band, and sample-versus-bulk rules on the PI, then repeating that weigh on a known scale. Send the rating list and kg or lb mark through the quote page.
Need a weight-tolerance line on your plate PI?
Send target ratings, kg or lb marks, and whether you need a pair delta. Yunlingyu will confirm the weighing method before bulk.
