Technical Problems In Measuring Nut Sizes
Technical Problems in Measuring Nut Sizes
- Nut sizes by the standard are referred to by their across flat measurement or AF. This is easy on an uninstalled nut. However, this gets a little dodgy when measuring across the flats of an installed nut. When a nut is installed, most procedures will require at least 3 threads are extending above the top face of the nut. These extending threads tend to block access from the measurement on the top. The stud also extends further than the corners of the nut flats and get in the way. Such that technician needs to “eyeball” or visually guess the proper measurement bend the ruler or use some form of trickery to get a measurement. Vernier calipers can reach across the flats without the stud getting in the way. However, they are too accurate as they measure in .0001” of an inch.
- The standard set by ASME has a minimum and maximum size envelope of acceptance. That is to say that the maximum of a 2” AF nut would measure exactly 2” (2.000”) but it has a minimum (Yet acceptable and called a 2” nut) size of about 1-15/16” (1.938”). This under-sizing of the nut can be frustrating if it is measured with a pair of Vernier calipers. Vernier calipers are very precise as they measure in thousands of an inch and are commonly used by machinists and inspection personnel. If they are expecting a 2” nut, it will seem to be off from the industry standard as they measure under-size of what is expected.
- Heavy Hex Metric and Imperial (fraction) size as set by ASME, ANSI, and ISO Common sizes are not what most would expect to find. Particularly if it is an installed nut with the stud in the way.
It is not uncommon that:
- 1-7/16” gets misinterpreted as 1-1/2” (as it is only 1/16” under)
- 1-13/16” gets misinterpreted as 1-3/4” (as it is only 1/16” over)
- 2-15/16” gets misinterpreted as 3” (as it is only 1/16 under)
- 4-5/8” gets misinterpreted as 4-1/2” (as it is only 1/8” over)
- To the end of accurately finding the correct nut size, the technician may use a tape or ruler which has increments of 1-1/8”, 1/16”, 1/32” or 1/64” thinking the more ticks, the more accurate it will be. However, this attempt at rigorous precision only serves to confuse the technician. He has about 20 standard Imperial Heavy Hex sizes and/or 25 standard Heavy Hex metric sizes but has to sift through all those tick marks to make a selection. Having a rule that only shows the Industry Standard Sizes makes for easier size identification.
- It is expensive when an incorrect nut size has been determined. On a bridge, reactor or wind turbine, it’s a long climb up. The wrench or hydraulic wrench set-up may weigh 130 lbs. Also, when the part of a job-scope where the nut is removed, if it isn’t removed, the job stops. Along with the other workers, support, safety, maintenance, engineering, crane operators, spotters, and assistants. All that preparation that went into getting ready for this job, Rent a Crane & operator, build scaffolding, volumes of permitting and planning, management of change. The Work Permits which are time stamped and have to be reissued at the end of each shift. Then there is the timeline, which is now extended; so the loss of uptime which can be an opportunity cost in the millions per day of lost production, for some plants. Particularly, if you’re the Technician with the wrench, who will turn the nut. The egg lands first on your face. It is very important to get the measurement right the first time.