Holding It All Together, Machine Screws, Hex Head Cap Screws or Bolts to Laymen

It’s funny, but one of the simplest devices we encounter has so much impact on whether things work or don’t.  Here in the EHT office, we have our favorite fastener companies, and they are the ones whose fasteners not only do what we expect them to, but actually outperform our expectations.  These products are few and seem to be getting fewer as time goes by.   A couple of years ago, Matt Weber and I were guests of John Deere, and the marketing people were talking about four wheel steering and other whiz bang features, I don’t remember all of what they said. I do remember seeing what we called D4C screws used in critical areas.  D4C stands for drilled for cotter pins. I realized someone in engineering had actually done something right in an area that he would probably not receive any awards for, or even recognition. But when the owner of the tractor didn’t have to chase down replacement screws that had vibrated off due to the nylon lock nuts failing from repeated tightening-fastening cycles, his forethought would be appreciated.

I was taken back to that day several years ago this weekend, when while assembling a variety of garden tractor accessories, I noticed that even in its consumer lines John Deere quality shows through in ways that only an engineer or mechanical engineering aficionado can appreciate. The Non JD products use plastic and lots of it and directions are hit or miss at their best. John Deere used zinc plated temporary spacers that would be discarded during installation. How many companies would go to this level? Frankly I could better deal with the imported products, if someone with a first grade sense of English and a sixth grade understanding of things engineering and process wrote the instruction manuals.

To this end, kudos to whoever was behind the instruction manual for the 17 cubic foot trailer for garden tractors sold through Lowe’s. The manual was well written and illustrated. The parts were blister packaged in order of installation and labeled to coincide with the assembly manual.  After spending eight hours putting together garden accessories this weekend, I was on the verge of resigning my job here and setting out to right what is wrong in the assembly manual publishing industry.  Then I realized the problem is bigger than what I, one man, could tackle.  But for those in the industry, actually have the people writing the manuals assemble the products using their instructions, or better yet, have a test group of laymen try and put together the product, just using the manual provided.

Well enough of the digression into the area of poorly written manuals, and back to fasteners.  It is fasteners that hold our machines together. This weekend was full of SAE washers, which weren’t really SAE washers by definition and bolts, screws, hitch pins, cotter pins and spacers.  There are people who are connoisseurs of wine. I am a connoisseur of fasteners. Upon graduating from college, I took a year off to work for a fastener company before I would go to law school.  The owner of the company was my best friend’s dad and when  he told his dad he should hire me, he guffawed at the idea. What possibly could a kid from the suburbs know about fasteners. So he asked me a few questions and what he didn’t know was that I had grown up in a house with a father who thought anything mechanical he could fix, redesign or make a piece work in an application that it wasn’t designed to.  So he hired me and that began my journey into the world of fastening.  What I took away from those seven years was an appreciation for well made fasteners and a knowledge of what should be used and where.

The basic fastener rules we run into around the house are grade 2, and grade 5.  Grade 2 are basic everyday mild steel bolts where very little loading is usually involved. Grade 5 cap screws are most commonly found in automotive applications and have a greater tensile strength than Grade 2.  If you look at the top of the cap screw, often called a bolt, you will see three slash marks to indicate a Grade 5 screw, none for a Grade 2.  Grade identification is easy, just add two to the number of slash marks found on the cap screw head and that is the grade on ferrous bolts. Grade 8 will have six slash marks. Stainless and non-ferrous metals use a different identification system.   Grade 8 cap screws are the bad boys of hex bolts, used in application where strength is critical.  Also the manufacturer’s stamp is found on the head.  I look for this stamp because I am familiar with the quality of products different companies produce.

How rust resistant is this screw? As in grades, there are a variety of finishes that provide rust resistance to screws.  We are all familiar with zinc plating, the chrome looking screws seen in home improvement stores, etc. These are not rust proof and are hardly rust resistant.  I think the real reason for the plating originally was to ease fastening these screws as the plating gave a slick finish to the screw. True weather resistant screws begin with hot dipped galvanized fasteners. These fasteners appear gray and dull.  Because of the chemicals involved in this process, these screws cost more than zinc plated. You will also see variations of zinc plating and dichromate finishes. I always thought the yellow dichromate were the most attractive finished screws. The yellow dichromate finish was used extensively in Grade 8 screws and  I still expect to see six slashes when I see yellow chrome.  Professional Tip: When buying Hot Dipped Galvanized Screws, always, always get HDG nuts and washers to be used with the HDG screws. HDG is a thicker finish and requires HDG nuts machined to a larger opening to accept the screw without removing the finish when threading into the nut.

Have you ever had problems trying to thread a nut and screw of the same diameter. The screw starts then immediately stops.  You have just discovered fine thread and coarse thread’s inability to be used together.  Coarse threads are the more frequently used cap screws.  You will see these markings on coarse thread packaging, 10-24, 1/4-20,3/8-16,1/2-13,3/4-10 and so forth. The first number indicates the screw diameter and the second number indicates the threads per inch.  On fine thread packaging the  information reads, 10-32, 1/4-28, 1/2-20 and so forth. Fine threads are usually used where vibration is present. The finer thread requires the nut to rotate more turns to move the same distance on a bolt.  Many of today fasteners use nylon lock nuts on coarse thread screws rather than use the fine thread patterns.  I have an opinion on this practice, and it involves the use of fine thread nylon lock nuts for added security. Also heat and nylon don’t play nicely together.  If there is heat in the application, use a castellated nut that accepts a cotter pin to lock it into place.  On larger application we used all-metal lock nuts for applications like those found on trains and large equipment.

I will briefly cover stainless and non-ferrous fasteners.  Stainless is the pinnacle of weather proof and affordable fasteners. Yes, I know titanium, cobalt and special unobtainium are out there, but the likelihood of us using these in everyday tasks is still remote.  18-8, and 304 stainless are the most common forms of stainless fasteners.  We used more 316 and B8m Stainless in our business, due to the paper mills that require a higher chromium content due to the acid nature of the environment involved in making paper.   We also did the sound system fasteners for IMAX sound, Disney World and other high tech installations done by a local sound company, that was once a  small operation working out of a storage unit. Those audio engineers set the standard for commercial installations.  They didn’t have excess money back in those days and instead of buying 316 stainless, they used 304 and 18-8 and we took large commercial magnets to our stainless stock and any fastener that wasn’t affected by the magnets, they used. The non magnetic fasteners would not be affected by the electrical currents in the speakers and electronics.

Other stainless types are Cor-Ten, used in furnaces along with 310 and 309 stainless. You will see 400 series used in knives etc.   Some of the stainless used in fasteners only exist in books until someone has the dollars to ask for it to be made.  At the time I didn’t know it, but I was asked to supply double ended studs in very large diameters for a Department of Defense job.  The material spec’d I found in a Carpenter Stainless Book.  630 stainless, ph1154 or something like that with a ton of other specs attached to the request.  I found a company in Cleveland, Ohio willing to obtain the material and machine the studs. These were very expensive pieces of steel. Months later when they showed, all of us wanted to see what something that expensive would look like. The studs appeared as though they had been left in the furnace too long and were blackish and nasty.  What they lacked in attractiveness they made up for in longevity and strength. No longer classified, those studs became the lynch pins on the blast tubes on a new class of submarines.

I will cover tapping screws, lags and drill point screws later.