Real versus nominal value

Last updated

The distinction between real value and nominal value occurs in many fields. From a philosophical viewpoint, nominal value represents an accepted condition, which is a goal or an approximation, as opposed to the real value, which is always present.

Contents

Measurement

In manufacturing, a nominal size or trade size is a size "in name only" used for identification. [1] The nominal size may not match any dimension of the product, but within the domain of that product the nominal size may correspond to a large number of highly standardized dimensions and tolerances.

Nominal sizes may be well-standardized across an industry, or may be proprietary to one manufacturer.

Applying the nominal size across domains requires understanding of the size systems in both areas; for example, someone wishing to select a drill bit to clear a "1⁄4-inch screw" may consult tables to show the proper drill bit size. Someone wishing to calculate the load capacity of a steel beam would have to consult tables to translate the nominal size of the beam into usable dimensions.

When considering the engineering tolerance between a shaft (or bolt) going through a hole in some other part (such as a nut), both the shaft (or bolt) have the same nominal size (also called the basic size), [2] [3] [4] but all the holes are physically larger and all the shafts are physically smaller in order that any shaft (or bolt) of a given nominal size can fit into any hole of the same nominal size.

In measurement, a nominal value is often a value existing in name only; [5] it is assigned as a convenient designation rather than calculated by data analysis or following usual rounding methods. The use of nominal values can be based on de facto standards or some technical standards.

All real measurements have some variation depending on the accuracy and precision of the test method and the measurement uncertainty. The use of reported values often involves engineering tolerances.

One way to consider this is that the real value often has the characteristics of an irrational number. In real-world measuring situations, improving the measurement technique will eventually begin yielding unpredictable least significant digits. For example, a 1-inch long gauge block will measure to be exactly 1 inch long until the measuring techniques reach a certain degree of precision. As techniques improve beyond this threshold, it will become clear that 1 inch is not the real value of the gauge block length, but some other number approximates it.

Examples

In various subfields of engineering, a nominal value is one for which the "name" for the value is close to, but not the same as, the actual value. Some examples:

In the United Kingdom, pipe is available that is quoted in both metric size and imperial size. The metric size is larger than the imperial size. For example, both 1⁄2 inch and 15 millimetres (0.59 in) copper pipe is actually the same pipe which has a nominal internal diameter of 1⁄2 an inch and a nominal external diameter of 15 millimetres [6] (diameter is always internal in the imperial measurement system and always external in metric).

A machine is designed to operate at some particular condition, often stated on the device's nameplate. For example, a pump is designed to deliver its nominal pressure and flow while operating at its nominal speed and power. Actual operating conditions may vary.

Other cases involve diameter, speed, and volume.

Sometimes the word "nominal" is misused in engineering contexts as a synonym for "normal" or "expected"; for example, The rotor resistances on all the other operating wheels are nominal. [8]

See also

Notes

  1. ↑ R. K. Rajputpage A textbook of manufacturing technology: (manufacturing processes), Firewall Media, 2008 ISBN   81-318-0244-2 page 705
  2. ↑ Coban Engineering. "Tolerancing Definitions".
  3. ↑ Bryan R. Fischer. "Mechanical Tolerance Stackup and Analysis, Second Edition". 2011 p. 410.
  4. ↑ Bruce J. Black, "Workshop Processes, Practices and Materials". 2015. p. 70.
  5. ↑ ASTM D3039, D4139, and others
  6. ↑ http://www.engineeringtoolbox.com/nps-nominal-pipe-sizes-d_45.html Nominal pipe sizes
  7. ↑ "Basic Tutorials: Solar Panels". Retrieved 2014-06-25.
  8. ↑ "Rear Wheel Trouble Continues". JPL. 2009-12-10. Retrieved 2009-12-10.

Related Research Articles

<span class="mw-page-title-main">United States customary units</span> System of units of measurement commonly used in the United States

United States customary units form a system of measurement units commonly used in the United States and most U.S. territories, since being standardized and adopted in 1832. The United States customary system developed from English units that were in use in the British Empire before the U.S. became an independent country. The United Kingdom's system of measures evolved by 1824 to create the imperial system, which was officially adopted in 1826, changing the definitions of some of its units. Consequently, while many U.S. units are essentially similar to their imperial counterparts, there are noticeable differences between the systems.

The board foot or board-foot is a unit of measurement for the volume of lumber in the United States and Canada. It equals the volume of a board that is one foot (30.5 cm) in length, one foot (30.5 cm) in width, and one inch (2.54 cm) in thickness. Board foot can be abbreviated as FBM, BDFT, or BF. A thousand board feet can be abbreviated as MFBM, MBFT, or MBF. Similarly, a million board feet can be abbreviated as MMFBM, MMBFT, or MMBF.

<span class="mw-page-title-main">Engineering tolerance</span> Permissible limit or limits of variation in engineering

Engineering tolerance is the permissible limit or limits of variation in:

  1. a physical dimension;
  2. a measured value or physical property of a material, manufactured object, system, or service;
  3. other measured values ;
  4. in engineering and safety, a physical distance or space (tolerance), as in a truck (lorry), train or boat under a bridge as well as a train in a tunnel ;
  5. in mechanical engineering, the space between a bolt and a nut or a hole, etc.

The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries. It has the same 60° profile as the ISO metric screw thread, but the characteristic dimensions of each UTS thread were chosen as an inch fraction rather than a millimeter value. The UTS is currently controlled by ASME/ANSI in the United States.

<span class="mw-page-title-main">National pipe thread</span> U.S. national technical standards for threads on pipes and pipe fittings

American National Standard Pipe Thread standards, often called national pipe thread standards for short, are United States national technical standards for screw threads used on threaded pipes and pipe fittings. They include both tapered and straight thread series for various purposes, including rigidity, pressure-tight sealing, or both. The types are named with a full name and an abbreviation, such as NPT, NPS, NPTF, or NPSC.

<span class="mw-page-title-main">Socket wrench</span> Lever with interchangeable socket heads to grip or turn a bolt or nut

A socket wrench is a type of spanner that uses a closed socket format, rather than a typical open wrench/spanner to turn a fastener, typically in the form of a nut or bolt.

<span class="mw-page-title-main">Screw thread</span> Helical structure used to convert between rotational and linear movement or force

A screw thread is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a threaded fastener.

British Standard Whitworth (BSW) is an imperial-unit-based screw thread standard, devised and specified by Joseph Whitworth in 1841 and later adopted as a British Standard. It was the world's first national screw thread standard, and is the basis for many other standards, such as BSF, BSP, BSCon, and BSCopper.

British Association screw threads, or BA screw threads, are a set of small screw threads, the largest being 0BA at 6 mm diameter. They were, and to some extent still are, used for miniature instruments and modelling.

<span class="mw-page-title-main">Trapezoidal thread form</span> Screw thread profiles with trapezoidal outlines

Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews. They offer high strength and ease of manufacture. They are typically found where large loads are required, as in a vise or the leadscrew of a lathe. Standardized variations include multiple-start threads, left-hand threads, and self-centering threads.

<span class="mw-page-title-main">Pipe (fluid conveyance)</span> Tubular section or hollow cylinder

A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases (fluids), slurries, powders and masses of small solids. It can also be used for structural applications; hollow pipe is far stiffer per unit weight than solid members.

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. "Nominal" refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number. Specific pipe is identified by pipe diameter and another non-dimensional number for wall thickness referred to as the Schedule. NPS is often incorrectly called National Pipe Size, due to confusion with the American standard for pipe threads, "national pipe straight", which also abbreviates as "NPS". The European and international designation equivalent to NPS is DN, in which sizes are measured in millimetres, see ISO 6708. The term NB is also frequently used interchangeably with DN.

British Standard Pipe (BSP) is a set of technical standards for screw threads that has been adopted internationally for interconnecting and sealing pipes and fittings by mating an external (male) thread with an internal (female) thread. It has been adopted as standard in plumbing and pipe fitting, except in North America, where NPT and related threads are used.

The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. They were one of the first international standards agreed when the International Organization for Standardization (ISO) was set up in 1947.

In engineering and machining, an allowance is a planned deviation between an exact dimension and a nominal or theoretical dimension, or between an intermediate-stage dimension and an intended final dimension. The unifying abstract concept is that a certain amount of difference allows for some known factor of compensation or interference. For example, an area of excess metal may be left because it is needed to complete subsequent machining. Common cases are listed below. An allowance, which is a planned deviation from an ideal, is contrasted with a tolerance, which accounts for expected but unplanned deviations.

<span class="mw-page-title-main">Screw</span> Type of fastener characterized by a thread wrapped around a cylinder core

A screw is an externally helical threaded fastener capable of being tightened or released by a twisting force (torque) to the head. The most common uses of screws are to hold objects together and there are many forms for a variety of materials. Screws might be inserted into holes in assembled parts or a screw may form its own thread. The difference between a screw and a bolt is that the latter is designed to be tightened or released by torquing a nut.

<span class="mw-page-title-main">Copper tubing</span> Type of metal tubing

Copper tubing is available in two basic types of tube—plumbing tube and air conditioning/refrigeration (ACR) tube, and in both drawn (hard) and annealed (soft) tempers. Because of its high level of corrosion resistance, it is used for water distribution systems, oil fuel transfer lines, non-flammable medical-gas systems, and as a refrigerant line in HVAC systems. Copper tubing is joined using flare connection, compression connection, pressed connection, or solder.

<span class="mw-page-title-main">U-bolt</span>

A U-bolt is a bolt in the shape of the letter U with screw threads on both ends.

<span class="mw-page-title-main">Wrench size</span>

Width across flats is the distance between two parallel surfaces on the head of a screw or bolt, or a nut, mostly for torque transmission by positive locking.

<span class="mw-page-title-main">Preferred metric sizes</span> Metricated industry standards

Preferred metric sizes are a set of international standards and de facto standards that are designed to make using the metric system easier and simpler, especially in engineering and construction practices. One of the methods used to arrive at these preferred sizes is the use of preferred numbers and convenient numbers, such as the Renard series and 1-2-5 series, to limit the number of different sizes of components needed.

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.