What You Will Learn
- How to read percent, gram-band, and pair-delta lines on a spec sheet
- Why bumper weight plates need mass and diameter as separate fields
- What conditions must sit next to a plus-or-minus number
- When a spec sheet is incomplete and should not be signed
Table of Contents
What a plate tolerance line is
A weight-tolerance line on a spec sheet is a written rule for how far a finished plate may sit from its marked rating, on a named scale, after a named preparation. It is not a slogan printed next to a photo. If the sheet does not say percent of marked weight, a gram band, or a pair delta, you cannot inspect the lot.
On bumper weight plates, buyers often mix two ratings: bumper plate weight and outer diameter. Those are different fields. A 450 mm plate can still miss the mass band. A plate inside the mass band can still be thin.
Why a bare plus-or-minus is not a spec
Catalog copy such as “plus or minus 2 percent” does not tell the inspector what to weigh, whether the plate is dry, or whether left and right plates in a set must match. Two factories can both print 2 percent and fail each other’s incoming check because one uses marked kg and the other uses a nominal lb conversion.
Rubber volume on a bumper also moves mass without changing the insert. If the sheet only lists insert steel, the rubber can run heavy or light and still “pass” a core-only note. Write finished-plate mass on the sheet the gym will receive.
Three ways factories write the band
| Method | How you calculate | Best use | Must also write |
|---|---|---|---|
| Percent of marked weight | (measured minus marked) divided by marked | Heavier plates where grams get large | Marked unit; dry; scale resolution |
| Gram or ounce band | measured minus marked, compared to a fixed band | Small change plates | The band in g or oz; same unit as the mark |
| Pair delta | difference between two plates of the same rating | Gyms that load matched pairs | How you pick the pair (same carton, same color) |
Pick one primary method per SKU family. You may add a pair delta. Do not let the warehouse switch from percent to grams because the first reading “looked close.” Bumper plate weight should be judged on the finished plate, not on a cut sample of rubber.
How to verify the sheet against a scale
- Copy the tolerance sentence onto the PI exactly. If the factory emails a new number, revise the PI; do not keep two versions.
- Photograph the mark and the scale display on the approved sample using the same method the bulk AQL will use.
- Confirm scale resolution is finer than the band. A 100 g scale cannot judge a 50 g band.
- Weigh dry unless the sheet says as-packed. Carton moisture is not plate mass.
- If diameter is on the same sheet, measure it with a stated tool and a stated point (outer rubber, not the hub). Do not use mass to “prove” diameter.
If the inspector cannot repeat the arithmetic from the sheet alone, the sheet is not ready for a PO.
Worked line: “Finished bumper weight plates, dry, marked kg, plus or minus 2 percent of marked weight, scale resolution 50 g or finer, pair delta X g for same-rating pairs.” That sentence is inspectable. “High quality, plus or minus 2 percent” is not. If the factory cannot put the first sentence on the PI, stop and rewrite the sheet.
What the number does not cover
Tolerance does not prove drop rating, hub insert weld, color coding, or carton grade. It does not convert kg marks to lb markets. If you need pound-marked plates, order that SKU. This page is not a laboratory standard. Attach a named method only if your retailer requires one.
FAQ
Can we accept plus or minus 2 percent on all bumper SKUs?
Only after you write the unit, the scale, and whether pair delta also applies. Treat 2 percent as a starting offer, not a universal law.
The sample is inside the band and bulk is not. Is the sheet wrong?
The sheet may be fine and the process drifted. Hold the lot, share scale photos, and ask whether coat volume or insert mix changed after the sample.
Should diameter and mass share one tolerance line?
No. Write two lines. A single “spec pass” box hides which field failed.
Do we need a third-party certificate?
Not for a routine PI method. You need a repeatable weigh and a written band. Use a lab if a contract names one.
Conclusion
A plate spec sheet is usable only when the band, the unit, and the weigh method are written so an inspector can fail a piece without a phone call. Send your rating list through the quote page.
Need the tolerance sentence on your plate PI?
Send ratings, kg or lb marks, and whether you need a pair delta. Yunlingyu will write a weigh method you can inspect.
