dho... this is a bit rambling, hope the main points get across...
Part of the problem of buying a part number [PN] from a vendor is that You are at the mercy of the vendor: They may/may not change the dimensions, materials/heat treatment, cold working, manufacturing process, etc over time... simply because the can... it's their part. However, IF the part fails to meet the vendors' own published dimensions and/or materials specifications, then shame on them for selling You an out of tolerance part. Suggest reviewing the vendors tech data sheet...the one Your purchased the part-against X-years ago and the currently available TDS on-line or in a brochure, etc... against the parts as-received. CAUTION: devil can be in the details [IE tolerances, options, materials, fab processes, etc], and there could be weasel words such 'as subject to change without notice', etc.
This applies for any vendor supplied commercial Item [Commercial off the Shelf - COTS or Commercial Item Description - CID] product: toilet paper, metals, plastics, adhesives, rivets, paint, cleaning compounds, fabric, etc, etc, etc. IF your item was made against a known spec then the vendor is obligated to have the item fully in compliance with that spec; otherwise it’s the wild-west.
Unless an 'item' is purchased against a known rigid specification, that requires certification testing, such as a federal, military, ASTM, SAE, etc spec, then variations are known to creep-in. Also, published specifications for vendor products are notoriously inaccurate about details; and may/may-not have any certification/quality verification testing done on-them. In many cases the TDS for a customer is bare bones and does not include essential manufacturing/conformity details.
NOTE.
Be extremely cautious if a vendor states that ‘PNxxxx has been inspected or tested [etc] per spec xxxxx’: This statement doesn’t mean it has been inspected, tested and certified to meet spec xxxxx… only that the vendor has picked/chosen elements of a spec to test their product against… and can subsequently use that spec tangentially [illegitimately] for their TDS/advertisement purposes.
Hmmmm... this situation is definitely NOT what we want in aerospace: unpredictable configuration and quality are positively unacceptable. It is essential for design/analysis/testing to validate a part ['item'] is suitable for use on an aircraft; then that item be purchased, installed and operated within known [not unknown limits]. To be of any use to us in aerospace a COTS item to be 100% predicable/testable/repeatable/reliable.
WARNING
Sub-tier vendors… especially those in the PRC… are a known to be extremely careless in their specification controls and fabrication documentation. I have seen this first hand: that is why current USA federal regulations state that anything installed-on, or used-in, an air vehicle, must have manufacturing traceability and certifications.
IF this item is not used in an aircraft or any flight related item... such as support equipment, tools or manufacturing... and strict controls are deemed important, then suggest You specify an item out of the catalog or TDS by part number; then repeat those critical elements [in this case dimensions, surface finishes, materials, spring-rate, etc] on Your purchase contract; or have them re-number Your item with a special identifier number to ensure the vendor is absolutely committed to produce that part; or procure that same spring from the vendor with Your drawing [P/N] of their spring with added details to ensure consistent manufacturing/quality… and indicate that ‘P/N xxxx’ may be purchased if it fully conforms to this drawing [company certification], then that PN would be acceptable’. OH yeah… and strongly suggest You mandate the vendor certify the shipped item as conforming to the TDS and contract or drawings, etc. IF a vendor will not comply with conformity documentations, then ‘Run-Forrest-Run”...
NOTE.
Spec-example of a Belleville spring…
MIL-W-12133/2 Washer, Spring-tension, Belleville Spring, Corrosion Resistant Steel
NOTE.
SAE design/usage manual has a lot of useful design, manufacturing, application info.
SAE HS 1582 Manual on Design and Manufacture of Coned Disk Springs (Belleville Springs) and Spring Washers
Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]