The fit and function of threaded fasteners is a complex topic, and rightfully so. Screw threads are one of engineering’s most fundamental joining methods, and regardless of the application, we expect threads to hold materials together tightly without failure. One of the metrics used to set the strength and reliability of a thread is its manufacturing tolerances – in other words, the allowable variance you can have in a given screw pattern and still consider it up to task.
Engineers specify threads by their manufacturing standards, referencing domestic or international codes, and each of these standards includes allowable tolerances governing the screw pattern. We’ll cover fastener standards in the next section, but first, let’s review what tolerances are and how they affect the performance of the fastener.
Thread patterns are described by their geometric profiles – internal or external thread; left or right hand; form; angle; pitch; and so on. Picture a normal machine screw: we see a triangular thread pattern that wraps in a helix around the bolt shaft, and we can see that the depth of the triangular cut, the spacing between threads, and other features are all uniform and consistent from top to bottom of the thread. This thread profile starts its life as a theoretical thread specification, where precise dimensions for all of these details are laid out with very exact numbers. Then, considering that these threads will be mass-produced by any number of manufacturers around the world, tolerance figures are added to each dimension, giving some freedom of variance while still assuring that the thread’s performance will be maintained.
Since threads are mostly employed as pairs between a male and female part (such as a bolt and nut pair), the tolerances for each part must be considered when they’re mated together. In other words, the tolerances of a male thread and the separate tolerances of its female mate must guarantee compatibility, assuring the performance of the fastener pair as a unit. To simplify all the different ways that fasteners can be paired, a Class Fit system is used to describe the fit between the two pieces. Thread Fit Classes cover the fit, play, and friction of a pair, working within tolerance ranges across thread diameters and surface finishes. Classes are known as 1, 2, 3 (loose to tight), are modified by adding A (external thread) or B (internal thread), and / or with stated special other systems such as H, L or U. The goal of the Class Fit system is to denote threads as interchangeable with each other.
Here’s an example: for a Class 2A ½”-13 bolt, the thread diameter has a tolerance of 0.4985” max to 0.4876” min, providing for manufacturing variation of 0.0109”. Not just for diameter, such tolerances apply to every measurable feature of the part.
Not sure which thread class or tolerances to specify for your project? E&T Fasteners offers expert opinions on these types of questions and more, helping you select the correct solution for your needs. To explore our hardware selection across all Class and Tolerance offerings, see our threaded fastener product selection here.
Thread Standards and Codes
Now that we know that tolerances and classes exist for thread pairs, how do we know which specifications to use? The answer depends on your locale, the application, requirements of your regulatory agencies, and engineering discretion over the performance required on the project. While there is no single global standard for threads, standardization work is ongoing and hopes to be there one day.
The first decision to make is if your thread will be in Inch units or Metric units. You’ll often see Inch units described with prefixes such as SAE or UC, and Metric units with prefixes such as M or SI. From there, common codes are as follows:
- Metric Units
- ISO Standard – the most common international standard with a variety of standard documents varying by application, such as ISO 724 for Metric Bolt Threads
- Inch Units
- UTS Standard – Unified Thread Standard is the primary code used in the US and Canada, and refers back to earlier ANSI/ASME B1.1 Unified Inch Screw Thread standards. This covers UNC Coarse, UNF Fine, UNEF Extra Fine, and UNS Special thread specs.
There are older standards that still carryover today in the way of stamping on bolts, nomenclature in marketing material, or old catalog numbers. A good example is the use of the SAE stamp on automotive bolts, referring to the Society of Automotive Engineer’s inch-unit thread standards, though they are in actuality using the UTS code.
Beyond these, let’s not forget that there are many more thread styles used today than those found in fasteners. A few examples: NPT and BSP are threaded pipe standards; GPI is a glass bottle and vial cap standard; RMS is a microscope objective lens thread standard. When unsure of a spec you come across, be sure to look it up, and never assume interchangeability even if the pair seems to fit together. The key to achieving intended performance is assuring compatible tolerances.
E&T Fasteners is here to help you navigate fastener options to best meet your application and project needs. Price quotes, availability, lead times, delivery confirmations, unique application suggestions – whatever your need, we’re here to take your call or email. For support and advice from our fully trained staff, speak with an E&T sales representative today.
About E&T Fasteners
With sales and support teams spanning the Continental US, E&T Fasteners is here to serve your hardware, fastening, and component needs both domestically and internationally. We are a stocking distributor of metallic, plastic, and exotic alloy fasteners, backed by our technical staff directly experienced in your unique applications. We can assist your specifying and selection efforts, providing engineered solutions, record-fast lead times, and stellar customer service from quote through delivery. Please let us know how we can help!