How do you recommend spare parts to customers

I am building an aisle crane with two large panels with standard parts like transformers, power supplies, breakers, contactors... and a controllogix plc with all ethernet connections. I am using SEW drives and SEW motors with encoders. Part of the job is to recommend spare parts to the customer. How do I decide which parts are storage worthy?

You can research how often different parts are generally needed to be replaced, but there is a large amount of variability for some of these things... primarily how hard they work the hardware and how good their power supply is for the plant.

Another factor would be, how important are the parts and how long are the lead times? Fuses are a dime a dozen, and you can pick one up from Lowes in an emergency. A servo drive that puts your whole line on a standstill is a different story. Better have at least one in stock just in case.

Thanks!

Thank you, LoganB! That seems like sound advice.

One more question though. The drives are usually in stock but the motors take 3 weeks to order. It seems like a no-brainer to have a spare for each except they are large and cost 2 thousand bucks plus per. Would you say yes or no to spares in that case?

I would base it on how much downtime the customer can afford if a motor goes out and the crane is out of commission until a new motor comes in.

In the stamping plants I get in downtime over a few minutes can be critical to shipping parts. If something can break and shut a machine, or worse a whole line, down then they will be sure to have one on hand, or they will be sure to have a local source readily available. When a press motor with an eddy-current clutch went out and it took a while to get another one they bought a spare $25,000 motor.

one thing I would consider in the design and build stage is what the customer
uses, brand names and drive sizes.
also what is their local distributor, ab, ge, Omron....

that way, if the system goes down and you didn't specify that part as a spare, they can go to the local distributor and find it quickly if the part is available.
as Aabeck pointed out, downtime can kill you. I know of a plant that is charged $10,000 per minute if production is delayed more than 15 minutes.
I would never have been able to stand that job pressure, so I turned it down when they offered me the job.
james

I use the rule of thumb that 10%, minimum 1, of active components should be spared.

This means that if there are ten or less of item A then you maintain one spare on site. Twenty or less then you keep two on site.

I don't bother sparing "inactive" components. Terminals, wire, mounting hardware, etc.

Lastly maintenance or "consumable" items, like fuses, batteries, etc. I usually go with 100% spares. i.e. I could replace every fuse once. Most likely these aren't big ticket items anyway.

Also as the designer, you should look to maximize commonality of parts. For example don't spec a 1.2KVA and a 1.5KVA drive or a 30W and 50W power supply. Just make both 1.5 KVA drives and both 50W power supplies. Any savings you might have will be eaten up by having to spare both.

This is what I would recommend. Now, they might be in for some sticker shock. At which time you can look at the lead time of the various items. Of course they need to compare this with cost of the equipment not working.

My typical sites use $1 per second for downtime. So, it doesn't take long to lose any savings by not carrying spares.

There are algorithms for estimating probabilities, risks, and costs associated with critical spares if you want to go down that route but you have to collect reliability information, lead times, downtime values, etc. to use them accurately which is why I think most people just use quick rules of thumb. I've done the calculations and generally they closely matched the recommended spare from the simple rule of thumb. Besides most of those calculation only really work well if you are deploying a fleet of machines.

The one thing I don't see very many people consider these days in obsolesce of the parts. One advantage of having a spare on hand is that if a device is obsoleted without direct replacement you have time to re-design before it breaks again (hopefully).

I would say see if you can walk their warehouse. What stuff do they *already* stock. Pushbuttons, contact blocks, what drives/motors do they keep on the shelf? Could YOU use those same replacements so they have one already there? I've never understood why companies don't do this. I mean, at the very least with pushbuttons and contact blocks as they are pretty common sources of failure.

If a part fails that is not on your list, how will that make you look to them?

If a part fails that is on your list, even if they choose not to stock it, you will at least be able to say they were warned. You may want to break it up into two priorities. High for things that are wear items likely to fail and/or long lead items, and a lower category for things that are unique to the machine but necessary for normal functioning.

I've seen this before and find that it's a sign the people buying the system aren't really competent to maintain it.



Mostly because stock management is dependent on a few variables that the OEM compiling said list may be unaware of such as local distributors...

cardosocea said:

I've seen this before and find that it's a sign the people buying the system aren't really competent to maintain it.



Mostly because stock management is dependent on a few variables that the OEM compiling said list may be unaware of such as local distributors...

Click to expand...

This. The company purchasing the equipment should already have an idea of how often pieces parts go bad on similar equipment they use. If they are buying a system that they have no experience with, they obviously haven't done any research.

The best advice for you is to err on the side of caution, as others have said, and recommend that they keep anything critical to the operation in stock as spares. If a fuse goes bad, you might not have one aspect of quality control or light that doesn't work until it's replaced. If you have a motor go bad, the entire piece of equipment may be down until it's replaced. If a lead time on the motor is 3 weeks, ask them how much it would cost for that piece of equipment to be down for 3 weeks. Usually, that helps alleviate any issues with sticker shock when you approach it that way.

And also as others stated, if you have a long list, at least when the components they decide NOT to stock go bad, you can say "told you so" which is far preferable to "why didn't you tell us this?"

Wow!

You all thought of this in ways I hadn't. Much appreciation!

Why not give them a list of everything with cost and lead time. Then let them decide what to keep? Or are they asking you what they should keep because they have no idea?
Do they even stock spare parts??
Them asking you to figure it out is sort of letting the fox in the hen house, especially if you are the one to provide the parts.
Typically, it doesn’t matter what the machine comes with. Most frequent flier parts will be exchanged for same but perhaps different manufacturers.

I would give them the top 5 grouped by lead time, then cost, then frequency. Let them figure out how much to keep.

When I would order parts for my last job since I did all the ordering I would keep 1-2 of each motor we used 3 of each cables like servo cables 2 of each servo amplifiers 50 of each push buttons since we used a few thousand of them and several E.STOPS since they seem to always break them 10 of each sensors and relays and overloads.