On the basis of temperature at which protruding portion of rivets are hammered, riveting can be grouped into two categories—hot riveting and cold riveting. In hot riveting, the rivet end is heated by some external means (like flame heating) before hammering. Heating temperature lies around 2/3^rd of melting point of the rivet material. Due to such heating, the material becomes soft and plastic and thus lower upsetting force is required. So when rivet material is hard, like stainless steel, hot riveting is preferred as lower force is required. It is also favorable for large diameter rivets, usually diameter larger than 10mm. Thermal expansion of rivet due to heating also has important role in gripping strength.

On the contrary, cold riveting is performed at room temperature only. Here rivet is not heated and thus hammering is carried out at room temperature. So comparatively higher force is required for upsetting; however, no heat source is desired for heating rivets. Heating time is also not associated with it, so the process is comparatively faster. However, if rivet diameter is large or it is made of stronger material, then large amount of hammering force is desired. Various differences between hot riveting and cold riveting are given here in table form.

Table: Differences between hot riveting and cold riveting

Heating the rivets and heat source: Hot riveting is performed by heating the protruding end of rivets to an elevated temperature (50 – 70% of melting point of rivet material) and thereafter upsetting that end to make another head. Thus it requires a pointed and high energy density heat source to quickly apply heat. Usually fuel or gas flame is utilized for such purpose. Cold riveting, however, does not require any heating for hammering and thus heat source is irrelevant to it.

Riveting time: Hot riveting takes more time due to heating of rivets just prior to hammering (upsetting). Cold riveting is performed at room temperature, so heating time is not associated to it. Thus it is faster, economic and more productive.

Stress on rivets, tightness and leakage: Rivet joint always experiences shear stress as force is usually applied along the components or perpendicular to the rivet axis. It is irrespective of the way riveting is performed (hot or cold). In fact, shear failure of rivets is important design criteria while designing rivet locations. Apart from shear stress, every rivet is also subjected to tensile stress when hot riveting is performed. This tensile stress is independent of external load on components. Like every material, rivets material also undergoes volumetric expansion during heating. Since free end of rivet is hammered at hot condition, so free contraction is restricted and thus tensile stress develops when it cools down. This tensile stress helps gripping components tightly in a leak-proof way. However, no such tensile stress is induced within rivets when cold riveting is performed as no heating is performed and thus no volumetric expansion or contraction is associated with it.

Hammering force: Engineering materials can be transformed from elastic state to plastic state either by applying sufficient force or by increasing temperature to certain level. A solid material in plastic state becomes very ductile and requires significantly smaller force (stress) for strain (change in dimension). In hot riveting, the rivet tail becomes plastic due to heating at elevated temperature and thus smaller hammering force is required. In cold riveting, the rivet material remains in elastic state as no heating is performed and thus comparatively higher hammering force is desired to bulge the tail.

When hot riveting is desired? If rivet material is hard, like stainless steel, then hot riveting is preferred otherwise large hammering force has to be applied. Consequently when rivet material is soft, like aluminum or brass, then cold riveting can be performed. From dimension point of view, if rivet diameter is larger than 10mm then hot riveting is recommended as it will require lesser force during upsetting. For smaller diameter rivets, cold riveting can be carried out.

Scientific comparison among hot riveting and cold riveting is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Difference Between Hot Riveting and Cold Riveting by difference.minaprem.com.
• Introduction to Machine Design by V. B. Bhandari (2017, McGraw Hill Education India Private Limited).

The post Difference Between Hot Riveting and Cold Riveting appeared first on Difference Box.

Temporary joining processes are all those joining processes that allow easy dismantling of joined components without rupturing them. It facilitates assembly and disassembly of solid structures as and when required without harming them. All fasteners basically provide temporary joints. Such joining is beneficial for inspection, maintenance and repair purposes as joint can be dismantled easily without breaking parts. However, they are prone to failure under vibration because of loosening of joining elements. Also strength of joint is not very high and thus these processes are not suitable for heavy load applications.

Such problems can be eliminated by utilizing a permanent joining process. By definition, a permanent joint is one that does not allow disassembly of joined components without rupturing them. Welding, riveting, coupling, etc. are common examples of such process. It can provide a sound, reliable, leak-proof and sufficiently strong joint and thus can be safely used for heavy load applications. However, it possesses problem during inspection and maintenance. Various differences between temporary joining and permanent joining are given below in table format.

Table: Differences between temporary joining and permanent joining

Possibility of dismantling assembled parts: As the name suggests, a temporary joint allow easy and quick disassembly of joined components without breaking them. Contrary to this, permanent joints are intended to stay in fixed position for longer duration. Such joints are permanent in nature and cannot be dismantled without breaking assembled parts.

Suitability: Since temporary joint allows disassembly without damaging parts, so it is preferred where frequent assembly and disassembly are desired. This makes replacing damaged parts easier. Interchangeability is one of the most crucial features offered by this joining. Permanent joining is suitable in those applications that require a strong, reliable and leak-proof joint and at the same time the joint is expected to stay in assembled condition for longer duration; for example window frame.

Strength of joint: Most temporary joining processes provide weaker joint that cannot bear higher load under service condition. Usually joining strength is considered to be lower than that of the parent components. Permanent joining processes usually provide sound, reliable and stronger joint. Joint strength is comparable with the strength of parent components and mostly these are equal.

Leak-proof joining: Permanent joining processes, if carried out properly using optimum process parameters, can provide leak-proof joints. This feature is very useful in many applications including pipe joining, pressure vessel joining, joining  of structural parts that are intended to stay within water as in case of ships, etc. However, the capability of sustaining pressure or force during service life varies greatly on different factors including joining process employed. Temporary joining processes do not usually provide leak-proof joints.

Biggest advantage of temporary joint: Since assembled parts can be dismantled without breaking them, so inspection, repair and maintenance are easier and at the same time cost and time efficient. Non-destructive testing (NDT) can be employed for inspection purposes. So no parts get damaged during testing or maintenance service. No permanent joint offers this advantage and thus destructive testing method is frequently employed for inspection and maintenance purposes. So parts get damaged and thus scrap rate increases and as a consequence overall cost of production also increases.

Examples of these joining processes: Fasteners are the fast and foremost example for temporary joints. It includes, but not limited to, threaded elements like nut, bolt, screw, etc., clips, buttons, clamps, cables, nails, hooks, rings, bands, staples, etc. Apart from fasteners, joining by press fit, cotter joint, knuckle joint, etc. are also temporary joining elements. On the other hand, welding is one overwhelmingly accepted permanent joining process. Except this, rivet joint, coupling, soldering, brazing, etc. are considered as permanent joining processes. Most adhesive bonding also affix parts permanently.

Scientific comparison among temporary joining and permanent joining is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

1. Difference Between Temporary Joint and Permanent Joint by difference.minaprem.com.

The post Difference Between Temporary Joining and Permanent Joining appeared first on Difference Box.

Both weld joint and rivet joint are considered as permanent joints; but their joining techniques, features of joint as well as application areas are different. By definition, welding is one type of permanent joining process by which two or more components can be joined together by coalescence formation with or without the application of heat, pressure and filler material. So here intended joining is obtained by the formation of weld bead or coalescence between two components.

On the other hand, riveting is also one permanent joining process where two components can be joined by means of long cylindrical rivets inserted through holes, which are drilled on the components prior to riveting. While welding requires edge preparation when plate thickness is more, riveting always requires pre-drilled holes on the component, which reduces load bearing capability of joined structure. However, performance of weld joint decreases when subjected to vibrations; but rivet joints exhibit excellent performance under same situation. Various differences between weld joint and rivet joint is provided below in table format.

Table: Differences between weld joint (welding) and rivet joint (riveting)

Requirement of holes on components: Welding requires no hole or slot to be made on the parent component; however, edge preparation is desired if component thickness is more. Such prepared edges are again filled by filler metal during welding. Riveting requires holes to be made on the component for passage of rivets. Such holes are actually weak portions and reduce overall strength of components, as discussed in the following section.

Strength of joint and load carrying capacity: A sound welded joint is considered to have 100% strength; in fact, strength of defect-free joint is more than that of the component. So welded assembly has same strength to that of the parent components. But riveted joints have significantly lower strength because of the presence of series of holes drilled on the components for passage of rivets. Such holes actually reduce resultant cross-sectional area of the assembled part. Rivets, itself, do not contribute in the overall strength of the joined structure. These areas are also mechanically weak portions due to immense stress concentration. Since strength is proportional to load carrying capacity of the solid member, load bearing capacity of welded structure is considerably higher than that of riveted structure.

Changes in metallurgical properties: Metallurgical changes are inherent to many welding processes mainly because of heating at an elevated temperature and subsequent cooling. Most fusion welding processes (such as arc welding, gas welding, resistance welding and intense energy beam welding) and certain solid state welding processes (where temperature rise is considerably high like friction welding) tend to alter various metallurgical properties like grain structure, grain orientation, level of crystalline defects, etc. Such changes can be observed on the weld bead as well as surrounding the bead in heat affected zone (HAZ). In most occasions, such changes are undesirable and disadvantageous. On the other hand, rivet joining does not affect metallurgical properties of the basic component as no heat is induced on parent metal.

Performance under vibration: Welded joints are prone to failure under incessant vibration. For this reason it is not preferred for various joining in bridge construction. However, with the extensive development of welding field in last few decades and emergence of many modern welding techniques, now weld joints can be applied for such applications without much problems. Rivet joints exhibit excellent performance under vibration and thus it is traditionally used in applications like bride construction, casing of machineries, etc.

Requirement of accessories and resultant weight: Rivet joint mandatorily requires additional strap plates, either in one side or in both sides of the joint. Such plates increase weight of overall structure. Rivets also contribute in increasing weight as a single rivet is heavier than that of component material removed by drilling for passage of rivets (because of rivet head and protruding end of shank). Welding, on the other hand, does not use additional plates and thus welded assemblies are lighter in weight. In homogeneous and heterogeneous welding methods, filler material is used; however, it only fills the root gap present in between two components and thus does not contribute in increasing structure weight.

Increase in dimension, joint appearance and sliding motion: Because of strap plates, rivet heads and hammered end in protruding part of shank, overall dimension of riveted assemblies increases significantly. Such protruding parts also hamper appearance and also restrict sliding motion on the surface. This sometime imposes restriction on its application. Contrary to this, defect-free welded assemblies offer apparently good joint with no changes in component dimensions. Reinforced metal, if present on joint due to application of excess filler metal, can be easily removed by grinding after the process. Apart from improving appearance, this also enhances sliding properties of the surface.

Time required for designing and processing: Design for welded assembly is easier as well as time and cost efficient. Welding process is also faster so it is more productive. Riveting requires lot of calculations to find out optimum number of rivets require, their size and position, etc. Thus designing is more complicated and time consuming. Moreover, drilling a number of holes on the component at exact locations and hammering of the protruding end of rivets require considerable amount of time, especially when it is directly performed by human operator. Thus riveting is also more time consuming than welding.

Possibility of joining various materials in different ways: Welding can be advantageously applied for joining a wide variety of materials including metals, ceramics, plastic and composites. A large number of welding processes exists to fulfill this requirement. Moreover, joining in different orientations like lap joining, butt joining, T-joining, cylindrical joining, etc. are possible by welding. Riveting is preferred for assembly of metallic materials in butt joining mode as lap joining requires additional plates that may not be feasible in all cases.

Areas of applications: With the extensive development of welding over last few decades, now-a-days a large number of welding processes exist which can be used for a wide variety of fabrication purposes. Its applications include, but not limited to, common household joining, automobile industries, electrical and electronic industries, civil construction, aerospace joining applications, etc. It can be profitably used for joining various shapes like sheets, plates, rods, pipes, etc. Compared to this, riveting has narrower field of application. Typical areas where riveting are commonly used are bride construction, pressure vessel, boiler, ship tussle, household frames, etc. However, now-a-days riveting is mostly superseded by welding as the later one offers a sound, strong, reliable and leak-proof joint.

Scientific comparison among weld joint and rivet joint is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.

• Welding Processes Handbook by Klas Weman (1^st edition, CRC Press).
• Springer Handbook of Mechanical Engineering, Volume 10 edited by K. H. Grote and E. K. Antonsson (Springer Science & Business Media).
• An Introduction to Metallurgical Laboratory Techniques by P. G. Ormandy (1^st edition, Pergamon Press).
• Welding and Joining of Aerospace Materials edited by M. C. Chaturvedi (1^st edition, Woodhead Publishing).

The post Difference Between Weld Joint and Rivet Joint appeared first on Difference Box.

What is joining?

Joining is one of the manufacturing processes that is used to assemble two or more solid components or structures together. It can be employed to permanently or temporarily join any type of engineering materials such as metals, ceramics, plastics and composites. In order to cater the need to join wide variety of materials in several ways, there exist various joining processes, which can be broadly classified into two groups—temporary joining processes and permanent joining processes.

Temporary joining—As the name suggests, a temporary joint is not fixed. That means it allows easy and quick dismantling of jointed structures without breaking them. This is very useful for inspection, maintenance and repairing purposes. However, such joint may not necessarily be strong and leak-proof. Typical joining processes that allow temporary joints include fastening, cotter joint, knuckle joint, key joint, press fit, seam joint, etc.

Permanent joining—A permanent joint is meant to stay fixed for long duration. It does not allow disassembly of jointed structures without breaking them. It possesses challenge during inspection, maintenance and repairing purposes as sometimes component or structure parts are required to damage. However, it offers a sound, reliable and leak-proof joint with high load carrying capacity. Typical permanent joining processes include welding, riveting, soldering, brazing, coupling, adhesive joining, etc.

What is welding?

As mentioned earlier, welding is one type of permanent joining process like riveting and coupling. It is one age old joining process that can efficiently and economically assemble two or more structural members permanently. By definition, welding is one of the joining processes by which two or more solid components can be joined permanently by coalescence formation with or without the application of external pressure, heat and filler metal.

As usual, there exist large number of welding processes for efficiently and reliably joining wide variety of materials in innumerable ways. Examples include arc welding processes (MMAW, GMAW, TIG, SAW, FCAW, ESW, etc.), gas welding processes (OAW, OHW, AAW, PGW, etc.), resistance welding processes (RSW, RSEW, PW, PEW, FW, etc.), solid state welding processes (DFW, PW, ROW, CW, FRW, FOW, etc.), and intense energy beam welding processes (PAW, LBM and EBW). Each of them offer certain advantages over others.

How welding differs from joining?

Welding is one type of joining process; others being riveting, soldering, adhesive, brazing, coupling, fastening, press fit, etc. It is worth mentioning that with the advancement of welding technique throughout last few decades, now-a-days welding mostly superseded other permanent processes including riveting. In industrial scale assembly purposes, fastening and welding are overwhelmingly employed. Therefore, joining is a generic term that indicates assembly of solid components; while, welding is one particular method of joining that can assemble two or more parts temporarily. Mathematically, it can be said that welding is one subset of joining set.

The post Difference Between Joining and Welding appeared first on Difference Box.