Need guidance on Mounting hardware 5

[Space213]

Hello, I have started my first engineering job recently and have been designing a unit that is driven by a Compressor and Pump. The compressor will be mounted vertically to a structural steel member that is supported by gussets. I have Gauges and a few digital meter readers that need to be mounted to sheet metal ( flush mounted ). My issue is what is the best and most efficient way to go about designing and picking out the proper screws and bolts? When should you install a washer to your design? My plan was to measure the holes of each hardware that need to be mounted and get a fastener that can fit into it and bolt it on from the other end. None of the hardware I have has any threads its all just one empty slot to put a screw through it.

Me being a new design engineer my designs are a bit lazy and sloppy because I am not aware or forget to include the tiny components that you never think about.

Any advice on how to go about finding the fasteners required for your own specific hardware if you were designing?

Thank you

 

[JohnRBaker]

If it's truly a 'slot' that will have either the head of the bolt or the nut butting up to it, then you need to put a least a standard flat washer under the head or nut. I was a machine designer for 14 years and this was one of the situations in which we almost always included a flat washer. The other common situation was when you had two or more holes, which were of different sizes and which you had no control over, through which a single fastener is installed, then a flat washer would be needed on the larger of the holes.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
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[Space213]

JohnRbaker,

how would you go about finding the right bolt to bolt down a Pump to a piece of iron for example? For example the mounting holes of the pump in the architecture design is listed at 0.432" so what size bolt/nut would you go for in mcmaster carr to make sure this design would work in the field when assembling?

 

[BrianPetersen]

Go down to the shop and see how they do it now with existing similar designs.

You need some actual wrenching experience ...

 

[Space213]

BrianPeterson,

I have already seen how its done. I have the basic wrenching experience its just to know how to find the right one based on the diameter of the mounting holes is the only thing I want to know how or how you would go about it if there is an easier more experienced way to find fasteners without wasting time

 

[MintJulep]

I'd look at something like this

https://www.boltdepot.com/fastener-information/bolts/US-Thread-Length.aspx

Then pick a 10mm bolt.

 

[BrianPetersen]

The bolt has to fit through the hole/slot with clearance, so the next common size below whatever the minimum tolerance dimension of the hole/slot is the one to get.

There is a fair argument that there should be a flat washer between any bolt/nut and whatever it is clamping, unless there is a specific reason for not having one.

 

[JohnRBaker]

For example the mounting holes of the pump in the architecture design is listed at 0.432" so what size bolt/nut would you go for...

Since you're quoting an Imperial dimension, I'm assuming that you're not looking at a Metric fastener (I could be wrong). If that's the case, I would opt for a 3/8 dia bolt, and in this case, I might also specify a flat washer although putting a 3/8 bolt into what's basically a 7/16 hole is pretty normal, even without a flat washer. But if you are using Metric fasteners, then MintJulep is correct, I would use a 10mm bolt and in that case, since it's a closer fit, there would be no need to even consider including a flat washer, as long at that 0.432 hole is actually a hole and not a slot.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[gwalkerb]

If you're mounting rotating equipment, even small stuff, the manufacturers probably have some recommendations about mounting their equipment. Some other considerations when selecting fasteners: do you have any alignment that needs to be done on the part during installation? You'll want to make sure you have some clearance between your fastener and the hole so that it's position can be tweaked, but not so much that's in an insufficient hold-down.

Depending on the size and power of the equipment, you might need to put some more care into material selection for your fasteners. Again, contacting the OEM is usually a good starting point to have confidence that you're mounting the equipment appropriately.

For things like gauges and other instruments, your company probably has some standard mounting designs, or if not standards, at least historical designs you can review. Supplying a mounting design that is what your shop is used to will be a good way to get off on the right foot and build a good working relationship.

 

[TimSchrader2]

Hello
Recommened a copy of the "Machinery's handbook" with most everything on bolts and fits and clearences. You could probaly find similiar info on line at the various bolt sites as mentioned above. But not as comprehensive.
.432-.375=.057 is too much clearence. Unless its a friction fit for alignment at install. Rule of thumb is about +.015 for that size. So its for a 10mm as mentioned.

 

[Jboggs]

Yes! Get your own copy of Machinery's Handbook as soon as you can afford it.

While I agree that .057 is about double the usual bolt hole clearance, I still think you should use a 3/8" bolt. The next larger size (7/16") is too big for the hole. Also, as a newbie you should always remember this - it is virtually impossible for a pattern of tapped holes in one piece and the pattern of clearance holes in the mating piece to be in perfect alignment with each other. That's why the clearance holes are larger than the fastener body, to allow the fastener to find its own center. The larger that clearance is the more important it would be to include a flat washer under the bolt head. Due to the vibration during operation, I would also include lock washers to prevent loosening.

 

[CWB1]

Spend some time researching proper bolted joint design. Until you understand how & why, dont take shortcuts, apply bad rules of thumb, or guess in engineering as suggested above. "An introduction to the design & behavior of bolted joints" by John Bickford is a good & common introduction most MEs will have an old copy of but there are others, including limited details in most machine design texts. Put together a spreadsheet or calculator and keep updating it as new joints and situations dictate, in mechanical design this will be one of your most common tasks.

Regarding the use of washers, that is a function of the fastener/nut head contact stress for the given joint at its design torque. If the stress is low then there's no need for a washer.

 

[drawoh]

Hello
Recommened a copy of the "Machinery's handbook" with most everything on bolts and fits and clearences. You could probaly find similiar info on line at the various bolt sites as mentioned above. But not as comprehensive.
.432-.375=.057 is too much clearence. Unless its a friction fit for alignment at install. Rule of thumb is about +.015 for that size. So its for a 10mm as mentioned.

A larger clearance allows for sloppier, more easily fabricated tolerances. Maybe he needs thicker, stiffer washers.

--
JHG

 

[drawoh]

Space213,

If you are just starting out in the business, there is a whole lot of practical stuff you do not know. I hope you are being mentored by an experienced engineer who can show you current practise and answer questions in context of where you are working. Learning your job by trial and error will involve lots of errors. The errors will affect your reputation.

You are ignorant, and you have an excuse for being ignorant. I presume you just graduated. Now is the time to ask stupid questions.

--
JHG

 

[Space213]

JohnRbaker,

The compressor actually has slots but the Pump has a hole but one side of the hole cuts through completely and is open if that makes any sense.

The rest of the pieces that I need to mount are just straight holes but very small. (Mounting small square shaped switches,air filters, etc)

What are the differences in mounting bolts to a slot rather than a symmetrical hole? I assume you would use a washer regardless to reduce any chances of a stress concentration and for a slot specifically I would mount the bolt right in the middle of the slot.

What would you say?

 

[Space213]

drawoh,

yes Its my first engineering job. and I am getting better with only a month in but still I am being told to be creative and handle it myself basically but its really hard to be creative with good ideas on things you have never designed before. Simply building stuff is something I have no experience with. The supervisors do not want me to ask too many questions because I can tell they rather just have me figure it out and show them what I have done. Its a start up company I work for so everybody has their hands full is why I assume. Love the job but its really not much training they promote me to just go online and re search. so that is why I am here.

 

[SwinnyGG]

Correct fastener selection is simple on its face, but it can get complicated very quickly. It is also one of the things that can lead shop personnel to greatly prefer working on the designs of one engineer over another- so you will serve yourself well to learn how to not only select fasteners which will do the job, but also make life as easy as possible for the people who have to build whatever it is that you design.

As a young engineer, NEVER forget this: you have a degree, and that's great. It does not, however, make you 'better' than anyone. The guys on your shop floor, or at your vendor, or at your customer, who build what you design are the ones who make the world go 'round. They very likely have decades of experience. Listen to them, and learn. If they challenge a decision you've made, ask yourself why. They may see a better solution than you did. If they are wrong, stand your ground, and explain your thought process to them so that they understand; in doing so, you may see an even better option than than either of you saw initially. If they are correct, make sure you learn from them and make sure they know that you value their input. This is, by far, the most important advice that anyone can give you in the first few years of the job.

With that said, in general, the decision tree for choosing fasteners goes something like this. Follow this and your fastener choices will be correct MOST of the time. You always need to take a step back after your design work is complete, before it is released, and make sure you haven't missed anything, but this will get you close.

Step 1) Choose a diameter

If one mating part has threads, use that size. Easy. If you have two clearance holes, find the hole size. Select the next smallest size fastener (a 1/2" bolt WILL NOT fit in a 1/2" hole- a 1/2" hole means a 7/16" or smaller fastener, etc).

If you see a hole callout with a 'strange' decimal value, this may indicate that the hole was drilled with a metric drill and a metric fastener is needed; convert your SAE dimension to Millimeters, and see if it's a match for a normal metric bolt clearance hole.

For example- 0.432 inches is 10.97mm. 11 mm is the correct clearance hole for an M10 bolt; so that part with a 0.432" clearance hole on the drawing was likely designed for M10 hardware.​

Step 2) Choose a style

What type of fastener and head do you want? Is this an assembly without much tool clearance, which would make a socket head cap screw easier to install than a hex bolt or cap screw? Is this a small/light weight component, which might only need machine or sheet metal screws for mounting?​

Step 3) Choose a length

Look at your stack-up of parts. Remember that with clearance holes, you will need enough thread on the far side for a washer and hex nut to properly seat; this means a minimum of 1 full thread must extend past the hex nut- if you use a nylock, past the nylon insert, etc. You want to use the shortest fasteners possible- so that your assembly will always be as inexpensive and as light as possible. If you are putting a fastener into a threaded part, in almost all cases any thread engagement beyond 2*bolt diameter is unecessary, so choose your length accordingly.​

Step 4) Choose a material and coating

Does your assembly have a unique corrosion requirement? Is it highly loaded (and thus require high-strength fasteners)? Does your customer have a specific cosmetic requirement? Answering these questions will point you at the material and finish type that you need.​

Now you have an 'ideal' fastener configuration for your particular application; in order to actually choose and provide part numbers on your BOM, you need to evaluate this choice and make sure it's correct.

A) Is the fastener I have selected easy to install? For example- if you've picked a hex head bolt, is there sufficient clearance around the bolt head AND nut for the tools necessary to actually install the fastener? If you've chosen a socket head cap screw- is there a direct path to the head of this screw so that an allen key or socket can easily be used to tighten it?

B) Is the fastener I have selected one that is easy to buy? For example- you may calculate that a particular application calls for a 1/2" SHCS, with a grip length of 3 1/8". 3 1/8" is not a standard length and will be hard for your purchasing people to find; the next step is to look at what options are readily available in the type/diameter/material/finish you need, and pick from that list. In this case: 3 1/8" length is not common, but 3 1/4" is very common, so choose 3 1/4" length. Now go back and make sure that this longer length does not collide with anything, still has enough tool clearance to be installed, etc.

C) Am I making life more difficult for the installer than I need to? For example- if you're mounting a bunch of electrical switches or pushbuttons, some with metric clearance holes and some with SAE- is there a single metric or SAE size that will work for all of them? Your panel builder will thank you if they can do the whole job with only one tool, instead of having to switch hex keys to hang two different parts which are right next to each other and appear to have the same hole size.

With regard to washers:

In my own practice, I use flat washers under bolt heads for every single fastener application with one exception- a SHCS in a counterbored hole does not get a washer; the clearance hole will be called out as a close fit, with no washer so that at the standard counterbore depth for the SHCS size chosen, the top of the SHCS is flush with the part surface.

For slots: if you put a high strength bolt into a slot, as the bolt is tightened, the face of the bolt contacting the part is very likely to round over the edge of the slot. Providing a washer prevents this. Also, a washer will generally be larger than the head of the bolt; this provides more area for the bolt to clamp against, which you need since much of the bolt clamping area the bolt would have in a normal hole has been removed to form the slot.

Lockwashers: ditch the standard helical lock washers that are common. They are useless, and their are papers and studies aplenty to back this up. Fastener loosening and what you can do to prevent it is a huge topic unto itself, but there are three main points which will serve you well in most situations:

1) By far, the best method to prevent fasteners from coming loose is making sure that they have enough preload. Bolts which are loose will easily become more loose; bolts which are sufficiently tight will stay that way for longer and under more severe conditions.

2) Helical lock washers simply do not work, ESPECIALLY when combined with a flat washer underneath, which is an extremely common configuration. Anyone who says otherwise has their head in the sand. Save your company and yourself time and money and don't spec them.

3) The most common reason for fasteners to come loose is not fasteners turning- it is loss of preload due to creep or relaxation of the parts being clamped. This is one reason for using flat washers everywhere; by spreading the clamping load of your bolt, you make it less likely that your clamped parts will 'squish' out from under the bolt head and cause your fasteners to come loose.

 

[JohnRBaker]

Where I worked, the rule was that whenever a fastener was used with a slot, that a flat washer was required. For simple holes, it was optional depending on several factors, most of which have already been mentioned either by me or others here.

And yes, the 'Machinery's Handbook' is a wise and useful investment. When I first started my professional career (1966, when I was still co-oping) I used a copy given to me by my father. It was the 14th edition, first published in 1953. Years later, when I could better afford the cost ($75 at the time), I purchased a copy of the 25th edition, published in 1996. I'm not sure what the current edition is as I retired almost 3 years ago and the 25th edition served me fine until then.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[JohnRBaker]

Some other things to consider...

If you work for a company that produces highly customized or one-of-kind machines this might not mean much, but if the products are part of a family of standard machines built for a single industry then it might be important to try and use standard, off-the-shelf hardware, items that are already in the conpany's inventory. Specifying odd-ball sizes or configurations, particularly if the number used is not large compared to other fasteners in use in your company, this can add addition costs and extra purchasing activity.

Also keep in mind that if the machine that you're designing is going to have to be maintained in the field on a regular basis by your customer's personnel, it might be a good idea, when practical and possible, to use as many common sizes as possible, even if it means 'over-engineering' a few fasteners so as to reduce both the number of tools needed when working on the machine and what the customer needs to keep in their inventory of spare parts.

And, as mentioned previously, the finish/material of the fasteners are also an issue. For example, the company I worked for when I working as a machine designer was manufacturing capital machinery for the food industry, specifically for large scale bakery operations. Since these machines were being used in the production of food, the materials that we could use, both for the structure and components of a machine, as well as the hardware, had to be safe for use where they could come in contact with food products. In our case, the biggest issue that the most common finish for nuts and bolts is Cadmium plated, which is what you get when you buy your screws at someplace like Home Depot or Ace Hardware. In our industry, Cadmium was verboten. We had to use things like Stainless Steel or more often, Zinc plated fasteners.

Also, again if it's practical, we also used two different classes for the bolts versus the nuts, like a standard size bolt with a heavy nut. That way the hex head's were of a different size so that the maintenance guy didn't have to have TWO of the same size wrench or sockets when he was loosening the fastened joint. Our machinery, while they were very expensive and complex, had to often be maintained by semi-skilled labor at best, and except for scheduled maintenance, most of the times that you had to work on something it was due to a breakdown, and unlike a normal production line, you can't just shut a bakery down for an hour or so to fix something as bread dough is a 'living' entity that if you don't keep to your production schedule, a stoppage of only 15 or 20 minutes can mean that an entire production run, upstream of the failure, has to thrown into the garbage bin.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[berkshire]

Space 213,
One tip to throw in the back of your mind, is this; Never use 1/4" diameter fasteners on product that is going to a metric country if the strength requirements are marginal. The reason is that the maintenance worker, if he loses your 1/4" bolt is going to replace it with a 6mm bolt. 6mm is .235" not .250" . Go next size up to 5/16" where the metric equivalent 8mm is a much better match.
B.E.

You are judged not by what you know, but by what you can do.