Drawing material callout: "material XYZ or Eq." 6

[just_some_shmuck]

For years, my company (consulting engineering) has included "or Eq." in drawing material callouts (IE "Material: ASTM A36 or Eq.")

Is this wise to do?

How will a fab shop interpret "equivalent"?

If I happen to suffer from momentary stupidity and specify a tube made from A36 I am fine if they substitute A500-B. If I specify a C-channel as A36 and they substitute A992-50 I am OK with that too. If I specify A514-B and they substitute A36 (much weaker), then I am NOT OK with that.

What are your thoughts on this?

 

[jmec87]

I think you are recognizing the risks with this - equivalency of different material grades is often unclear and could be interpreted differently by various parties.

Advantages of including "or Eq.":
- possibly reduced costs and/or lead times
- less engineering effort required to correctly specify materials and address deviation requests

Disadvantages of including "or Eq.":
- lack of control over materials used
- lack of fabricator accountability for using the right material
- risk of failure if unsuitable material is used (i.e. fabricator thought it was equivalent but it wasn't)

Considering the above, I think you're better off avoiding "or Eq." unless you really don't care what they use (i.e. for a part that won't be welded or significantly loaded). If you want them to have multiple material options, then list the allowable material options, and don't leave it to the fabricator to guess which materials could be equivalent.

 

[rb1957]

How is someone to determine what is "equivalent" ? It is better to define "acceptable alternatives".

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SwinnyGG]

Gotta be careful about that.. you could have a real disaster due to something as simple as someone using an NBR O-ring where you've specified Viton, because they thought that was equivalent. (..or whatever, you could think up a thousand examples)

I'd suggest modifying the note to state 'or approved eq' at minimum, so at least you have some level of protection by setting the expectation that questions need to be asked before material substitutions are allowed.

Best practice would be to have confirmation of material substitutions captured in whatever contracts you have with vendors, and not leaving it up to what's on drawings.

 

[3DDave]

The note "Material alternates require approval of waiver prior to production." would be my leaning.

 

[geesaman.d]

I have come to generally despise vendors deciding what is 'or eq' for me. As suppliers become more global this practice becomes even more risky.

In the world of pressure containing components, that's a death trap because 'or eq' may fall short on numerous critical physical parameters. At best, it's a system that's out of control.

At a minimum I would take off the 'or eq' and say "equivalents may be proposed and must be pre-approved by XXXX Engineering" and then have a process that helps you keep track of the equivalents used on each purchase order.

That said, overkill is not far away. I have customers who buy mostly piping and tanks and their precious little heads explode when I say I cannot and will not provide ASTM designations for "all materials" in the contract. (We only agree to that for pressure containing and process facing components only). If you're working with a variety of raw metal shapes, then keeping track of every ASTM designation for every piece of raw material is a lot of busywork. It can help to make a corporate standard for "low carbon steel" that lists the popular and appropriate ASTM, JIS, DIN, EN, etc standards for all of the common forms, so you can then reference that standard on your drawings.

 

[ctopher]

I would not use "or eq". That's like only calling out "Alum Alloy". They have freedom to do what they want.

Chris, CSWP
SolidWorks
ctophers home

 

[SWComposites]

Is this wise to do? No, completely no, not wise, nor a good idea. Unless you want to put your name and phone number next to the callout, like "or Eq that is prior approved by NAME (phone#)"

you are running a huge risk that some unqualified "genius" is going to make the "equivalent" decision, which will likely only be based on cost/schedule and a superficial at best understanding of the material.

 

[MintJulep]

When you, as the designer of something select a material you should have a comprehensive list of the material properties that are important for your specific application and the allowable range of values of each property that make a specified material acceptable.

That's the same list you would use to evaluate "or equal" against.

If you know that several materials would be acceptable to you, then list the acceptable materials.

Do not add "or equal". Vendors will anyways ask "is it ok to use....?". Without "or equal" you can simply answer "No", or make the evaluation, as you choose. If you include "or equal" then you are obligated to make the evaluation.

 

[mfgenggear]

Yep agree with all, no no no

 

[JStephen]

Generally, I would avoid the "or equal" whenever the design of that part assumes specific values for yield stress, etc.
On occasion, on our drawings, we'll wind up with something like "McMaster-Carr Part No. xyz123 or equal". It's easy to look it up on the McMaster-Carr site, but it's liable to be easier for the purchaser to pick up something similar at Lowe's or Grainger's, so we throw the "or equal" in.
A problematic item where this question might come up is bolts- spec says A325, is Grade 5 or Grade 8 acceptable?
There are some applications where the designs assume lower yield values, and higher yield values won't work for that design- true of some flange design/bolt design (where using a higher strength bolt will make the flange overstressed per the design method) or anchor bolts (where lower-yield bolts are assumed to yield, and that assumption fails if higher strength is substituted).

Another part of this question is that it becomes incumbent on the specifier to keep up with what is actually currently available. It is not uncommon for us to see specifications reflecting materials that were available 40 years ago and seldom used now.

 

[mfgenggear]

The normal procedure will be or should be
For the supplier or factory to contact the buyer or liason engineer if in house.
If material is no longer available for small lot. A expensive mill run is required.
Is contact materials engineering and get
An authorized substitution. Such as material spec has changed and no longer available.

Don't let the factory or supplier decide what is acceptable. The liability can be an issue.

 

[Christine74]

In some situations specifying "XYZ or Equal" or the general material grade can save you a lot of headaches down the road.

Even with Pressure Vessels where the material properties are absolutely essential, there can be a lot of miscellaneous components where the exact material grade is of little or no consequence. Examples include insulation support rings, manway davit arms, name plate brackets, earthing lugs, fireproofing support studs, etc.


-Christine

 

[drawoh]

Christine74,

If you don't care about the material, specify "CARBON STEEL". If you docare, specify the grade, and specify some quality control test you will do. Hardness?

I recently designed a spring for somebody. I analyzed for fatigue in the functional deflections, then I worked out a test of single deflection, considerably more than the functioal bending. This allows our inspectors to verify that the vendor used fully hardened material.

--
JHG

 

[Orange_kun]

I try to keep my material requirements as simple as possible. I usually specify with UNS values. If it's something I feel the manufacturer may have a hard time obtaining (shape or grade), I'll write out acceptable alternatives in a note. If even that isn't broad enough, there will be another note telling them to contact engineering for approval.

If I ask for something to be made from 17-4 PH in H900 condition, and all you can source is annealed stuff? The note will specify if pre- or post-machining heat treatment is permitted.

But generally, if you're asking for a material that is difficult to obtain there should be a good reason for it. "Equivalent" probably doesn't exist in an exact capacity. At which point you should have already reevaluated what you needed from the material and chosen something more common.

 

[mfgenggear]

Ya I guess if it was some commercial work when the only issue would.the product did not work and nobody was hurt except the user wallet.
I been in aerospace to long, peoples lives matter. Failure is not an option.
When bean counters get involved then people get hurt.

 

[Heaviside1925]

The companies I've worked with commonly use this type of phrasing on drawings. It serves two purposes. The first is to communicate to your customer what material was used as your design basis. You are not telling your customer they are required to use that material but that your design used that material to perform all the required checks and calculations for the design.
The second thing it does is saves you time and drawing rework down the road. This is where a material requestion/specification and drawing may differ. The material req or specification should include the requested material of construction and if the customer wants, includes phasing such as "equivalent materials of construction may be considered with the consent of the customer" So if the customer chooses to allow a vendor to substitute an equivalent material then your drawings need not change and the liability for that change is now your customers. Also, if this is being bid by multiple vendors it prevents the need for vendor specific drawings being created.
By the end of the project, the deliverables your customer should have is a final specification for the fabricated item stating what material were used and a set of drawings for the fabrication of the item. If down the road your customer wants another one of these items, they have the drawings showing the fabrication along with your design basis stating the material used for the design. So, any substitution of material is the liability of the customer and not you.

I will qualify all this by saying, if your design requires a certain material of construction and you are confident that any substitution could result in some sort of failure then you should communicate this to your customer. If your customer is aligned that only the material you specified should be used, then that needs to be communicated on the drawings as well, i.e. "substitution of materials of construction is NOT permitted".

 

[mfgenggear]

Hevi
Well written

 

[CWB1]

If I saw that on an internal print I'd be having a serious discussion about print quality with the checking department. On an external print, I'd be looking for a new supplier.

Material callouts should always be specific. Requesting a deviation only requires a few minutes of the supplier's time and approval only a few minutes of the engineer's (analysis excluded).

As written, your supplier is free to ship you any material they deem equivalent. Your quality dept cannot reject it, your purchasing dept must pay for it, and your shop may use it in a safety critical application. Bc your design documentation is imprecise you also wont win any sort of legal or regulatory challenge, and may lose expensive quality certs bc of it.

 

[SwinnyGG]

The companies I've worked with..........

Your post sounds as if you frequently design things for some 'customer' and then give them fully formed drawings to make things themselves. If so, that's great...

But in typical context, which I believe to be MUCH more common for the professionals on this forum, that's not what happens. Your 'customer' doesn't pay you for a set of drawings; they pay you for a finished product that functions per their specifications.

Allowing a vendor to change materials because you use a very loose drawing note to specify material in order to 'save drawing rework down the road' is a bug, not a feature.

I would never, ever put myself in a position where the performance of my parts are contingent on material choices made by my 'customer', who in my experience either has no understanding at all of what the impacts of a material change may be other than cost, or has that knowledge but is extremely cost driven to the point of sacrificing performance. Frequently it's both, particularly if your point of contact is someone in the purchasing silo and not an actual experienced engineer.