Machines are often built with a welded or bolted steel frame as a base structure. Today, however, more and more machines are built of aluminum extrusion or pipe-based structures. There are also many small structural assembly components available from machine part vendors.
Electrical components must be kept in an enclosed space to protect components or prevent personnel from contact with voltages.
There are a number of classifications associated with this protection.
8.1 Steel Framing
Most steel frames are welded for rigidity and permanence. Steel tubing, flat-stock, and angle pieces are cut to length and welded together, generally using a jig, clamp, or fixture to ensure alignment.
Because steel rusts or oxidizes, it usually has to be cleaned by grinding before being primed and painted. For food-processing or medical applications, frames are often made of stainless steel to eliminate the need for paint, which can cause contamination.
Premade welded steel bases can be purchased as a standard product from various manufacturers. These generally consist of a welded tube steel base, a ground steel top plate for mounting components, and adjustable feet and holes for bolting them to the floor (leveling feet). An example is shown in FIG. 68.
FIG. 68 Welded machine base.
Frames may also be bolted together. This is often done for one of two reasons. One, the relative cost compared with welding is much cheaper; two, the ability to unbolt the frame later makes for easy transport or modification. Using fasteners to build a frame is not as desirable as welding, especially on heavy-duty frames that may be subject to vibration.
If components are to be mounted to a frame, a piece of steel is often welded to the frame to be machined to a specific thickness afterward. This machine pad can then be ground, drilled, and tapped for precise location of a component or subassembly.
During the welding and machining process, stresses are induced into the welded frame. Prior to assembly, these stresses are often relieved by heating the frame in an oven or attaching a stress-relieving vibrator to the frame. This is especially important on larger frames or frames that have critical dimensions.
Shims
A shim is a thin piece of material used to fill a space or increase a dimension by a slight amount. It is used as an adjustment method and is often placed between two objects that have been bolted together as a spacer.
Shim stock can be purchased in varying widths and thicknesses to be cut into desired sizes. For industrial machinery, shim stock is usually made of metal, although plastic composites are sometimes used. Shim stock is also available in a laminated foil, which can be peeled off a stack to build up a surface.
Dowels and Dowel Pins
A dowel or dowel pin is a solid cylindrical piece of material that can be pressed into a hole as a locating device. Dowels in industrial machinery are usually made of hardened steel. Dowel material is usually machined to a very tight tolerance. This material is available in long sections called dowel rods, which are then cut into pins. The ends of the pins are then usually lightly chamfered. FIG. 69 shows a pair of dowel pins.
FIG. 69 Dowel pins.
Dowel pins may have a slightly smaller diameter than the hole into which they are inserted so that they float freely, or they may have the same diameter as the hole, which is then reamed for a press fit.
Using bolts as fasteners introduces mechanical play into the positioning of the mounted piece. Typically, fasteners have clearances because of the oversizing of the through holes. This is proportional to the size of the fastener. By using dowels for precise location, this mechanical play can be reduced by up to ten times or eliminated altogether. The use of dowels does often increase the amount of assembly and disassembly time for components, but when precision is required, it is well worth the added cost.
Dowels are not meant to provide structural support. Any more than two dowels in a single assembly is not recommended. As soon as a side load is applied a few times, the brunt of the force will be transferred to one of the dowels, making the others redundant. It also makes it very difficult to reassemble the pieces if they are ever taken apart. Fasteners or keys and slots should be used to take the side load forces. Dowels should be used only as a precision locating method.
FIG. 70 Keyed motor shaft.
Keys, Keyseats, and Fixture Keys
A key is used to transmit torque between a shaft and hub. If both shaft and hub have a rectangular slot or keyseat placed in the axial direction, a key can be assembled into the grooves, providing a positive surface for the motor shaft to exert force against.
Keyseats or keyways are sized based on the diameter of the shaft in both width and depth. ANSI provides a table for sizing the key size and keyseat depth. Key stock and bar stock are available in standard or metric dimensions, and their cross sections may be rectangular or square-shaped.
FIG. 70 shows a keyed motor shaft with a key inserted.
Since this key is captured (that is, the keyway does not extend to the end of the shaft), it must be inserted before sliding it into a coupling.
Fixture keys are used to accurately locate fixtures or machine components. Typically, they are assembled into a hole or a machined slot. There are many different types of fixture keys, from simple square or rectangular keys inserted into slots in two mating surfaces ( FIG. 71) to stepped or sine-type keys. They are made in metric or standard dimensions, usually of hardened steel. Socket head screws are sometimes threaded into the key for fastening.
Unlike dowels, fixture keys are designed to spread a load over a surface and are, therefore, appropriate for perpendicular support, that is, "side loading."
FIG. 71 Fixture key.
Machine Pads and Grind Spacers
Machine pads are pieces of flat stock that are welded to a frame in order to have something mounted to them. They often have tapped or through holes to accommodate the bolts for the mating assembly.
There are two important reasons for using machine pads rather than mounting directly to a frame: to provide thickness for tapping or structural integrity and to provide a surface that can be ground to make it flat and parallel to other surfaces.
For large assemblies that are to be mounted onto a machine pad or other flat surface, grind spacers are often designed into a machine.
These are pieces of metal that are purposely inserted between the flat machined surface on a frame and an assembly. This allows them to be ground to different thicknesses to make up for small differences in parallel surfaces. Rather than having to grind a large assembly or pads on a frame, the spacers can be easily removed and ground individually.
Fasteners
Fasteners are any device used to attach two pieces of material together.They include such classifications as rivets, bolts, and screws.
A rivet is a fastener used to attach items permanently. There are several different types of rivets including solid, tubular, or blind rivets-also known as "pop" rivets. Rivets consist of a shaft with a head on one end and are deformed after being inserted through a hole that passes through both pieces to be joined. This creates a connection that must be ground or drilled to remove. Solid rivets are typically deformed with a hammer, a rivet compression tool, or a crimping tool, which may be hydraulic, pneumatic, or electromagnetic. These types of riveting methods are used near the edge of the fastened materials so that the riveting tool can access both sides of the rivet.
For connections made away from the edge of the materials, a blind rivet, as shown in FIG. 72, is used. This is a tubular rivet with a large head that has a shaft with a mandrel through the center. The rivet is inserted through the hole and a special tool is used to pull the mandrel head into the rivet. The mandrel is designed to break off when enough force is applied.
Rivets may be made of steel, aluminum, or various other metals.
In industrial automation, one of the most common uses of rivets is to attach wireway or other components permanently to an electrical backplane.
A bolt or screw is a fastener with a threaded shaft with a head on one end that is used to apply torque, driving the fastener into a threaded hole or a nut. Screws are usually used to drive into a hole without the use of a nut and often create their own threads in the hole, whereas bolts may be threaded into a prethreaded hole or into a nut.
Bolt and screw heads come in many different shapes, depending on the type of tool that is used to drive them. Socket head and hex head are common forms of bolts, while Phillips or slotted heads are most common with screws. Screw heads can be pan or dome-shaped, round, countersunk, or several other forms. Special forms of bolt or screw heads also include hex socket, Robertson-or square drive-Torx, spanner head, and a variety of so-called "security" heads.
FIG. 72 Blind or pop rivet.
Screws and bolts are usually threaded in a right-handed direction, meaning that the fastener must be turned clockwise to tighten and counterclockwise to loosen it. Sizes of bolts and screws generally fall into standard (SAE) and metric dimensions, which specify the shaft diameter and thread pitch.
8.2 Aluminum Extrusion
Profiles of extruded aluminum are frequently used for machine guarding but can also be used to build machines of substantial size.
Aluminum sections are available in both metric and standard sizes from a variety of manufacturers. There is a wide range of available accessories, such as brackets, fasteners, hinges, plastic covers, and caps. Profiles are square or rectangular in their cross sections and include a "T-slot" in the side for fasteners, panels, or the routing of cables or hoses. Common vendors of these extrusion products include 80/20, Item and Bosch. FIG. 73 illustrates hardware for joining two pieces of aluminum extrusion.
Aluminum extrusion also comes in a variety of different colors, although the natural silver/gray anodized color is most common.
While aluminum profile is more expensive than an equivalent length of tube steel, this is offset by the cost of welding, painting, and labor. Aluminum is not subject to rust and is usually anodized for hardness and electrical resistance.
FIG. 73 Aluminum extrusion. (Courtesy of Bosch.)
8.3 Piping and Other Structural Systems
For lighter-duty applications, structures may be built of threaded pipe, angle, and flat stock. As with aluminum extrusion, these systems are sold by several different companies. Creform is probably the best known of the threaded pipe systems. These types of structural systems are often used for carts, roller conveyor stands, and racks. Automation elements, such as pick-to-light sensors and gravity conveyor rollers, can also be mounted using accessories from these systems. FIG. 74 shows a bin rack made of threaded pipe.
On a smaller scale, other vendors, such as Misumi, make structural items to mount sensors, gauges, or other devices. These items, sometimes known as "tinkertoys" (not to be confused with the wooden toy of the same name), are available in metric and standard sizes. Small brackets, bearings, gearing systems, and various other items are also available from this manufacturer.
FIG. 74 Pipe structure rack. (Courtesy of Creform.)
8.4 Electrical Enclosures and Ratings
Control and electrical components are usually housed in metal or nonmetallic enclosures. Enclosures may be made of steel, galvanized sheet metal, fiberglass, or plastic. These enclosures are available in a wide range of sizes and configurations and from various manufacturers. They typically have either screw covers or hinged doors for access to internal components and wiring.
Sizes range from small junction boxes or push-button boxes to large multidoor enclosures. Enclosures are rated for suitability for different environments by NEMA and IEC, as explained in the next section. A NEMA 12 enclosure is shown in FIG. 75. Larger enclosures are often manufactured with holes to mount a flange-mounted disconnect on the side by the door. These may be specified for disconnects from Allen-Bradley, Square D, Cutler-Hammer, or other major manufacturers.
Other accessories, such as filtered vents, fans, or even air conditioners, are available from enclosure manufacturers. Fluorescent lighting that is activated when a door is opened is another standard accessory.
Most enclosures are manufactured with studs to hold a metal backplane to mount components on. Backplanes may be steel or galvanized metal and are usually grounded during panel fabrication.
Well-known electrical enclosure manufacturers include Hoffman and Rittal.
FIG. 75 NEMA 12 electrical enclosure.
NEMA Ratings
Electrical enclosures are given a rating by NEMA. The following definitions are from NEMA Standards Publication 250-2003, "Enclosures for Electrical Equipment (100 Volts Maximum)":
In nonhazardous locations, the specific enclosure types, their applications, and the environmental conditions they are designed to protect against, when completely and properly installed, are as follows:
Type 1: Enclosures constructed for indoor use to provide a degree of protection to personnel against access to hazardous parts and to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt).
Type 2: Enclosures constructed for indoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt); and to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (dripping and light splashing).
Type 3: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow); and that will be undamaged by the external formation of ice on the enclosure.
Type 3R: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow); and that will be undamaged by the external formation of ice on the enclosure.
Type 3S: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow); and for which the external mechanism(s)
remain operable when ice laden.
Type 3X: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow); that provides an additional level of protection against corrosion and that will be undamaged by the external formation of ice on the enclosure.
Type 3RX: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow); that will be undamaged by the external formation of ice on the enclosure that provides an additional level of protection against corrosion; and that will be undamaged by the external formation of ice on the enclosure.
Type 3SX: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow); that provides an additional level of protection against corrosion; and for which the external mechanism(s) remain operable when ice laden.
Type 4: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow, splashing water, and hose directed water); and that will be undamaged by the external formation of ice on the enclosure.
Type 4X: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow, splashing water, and hose directed water); that provides an additional level of protection against corrosion; and that will be undamaged by the external formation of ice on the enclosure.
Type 5: Enclosures constructed for indoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and settling airborne dust, lint, fibers, and flyings); and to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (dripping and light splashing).
Type 6: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (hose directed water and the entry of water during occasional temporary submersion at a limited depth); and that will be undamaged by the external formation of ice on the enclosure.
Type 6P: Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (hose directed water and the entry of water during prolonged submersion at a limited depth); that provides an additional level of protection against corrosion and that will be undamaged by the external formation of ice on the enclosure.
Type 12: Enclosures constructed (without knockouts) for indoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and circulating dust, lint, fibers, and flyings); and to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (dripping and light splashing).
Type 12K: Enclosures constructed (with knockouts) for indoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and circulating dust, lint, fibers, and flyings); and to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (dripping and light splashing).
Type 13: Enclosures constructed for indoor use to provide a degree of protection to personnel against access to hazardous parts; to pro vide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and circulating dust, lint, fibers, and flyings); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (dripping and light splashing); and to provide a degree of protection against the spraying, splashing, and seepage of oil and noncorrosive coolants.
In hazardous locations, when completely and properly installed and maintained, Type 7 and 10 enclosures are designed to contain an internal explosion without causing an external hazard. Type 8 enclosures are designed to prevent combustion through the use of oil-immersed equipment. Type 9 enclosures are designed to prevent the ignition of combustible dust.
Type 7: Enclosures constructed for indoor use in hazardous (classified) locations classified as Class I, Division 1, Groups A, B, C, or D as defined in NFPA 70.
Type 8: Enclosures constructed for either indoor or outdoor use in hazardous (classified) locations classified as Class I, Division 1, Groups A, B, C, and D as defined in NFPA 70.
Type 9: Enclosures constructed for indoor use in hazardous (classified) locations classified as Class II, Division 1, Groups E, F, or G as defined in NFPA 70.
Type 10: Enclosures constructed to meet the requirements of the Mine Safety and Health Administration, 30 CFR, Part 18.
Tables from NEMA 250-2003 are contained in App. D.
IP Code Minimum NEMA Enclosure Rating to Satisfy IP Code TABLE 2 IP/NEMA Equivalents
IEC and IP Ratings
The IEC also rates both enclosures and other electrical devices for suitability in various environments. A rating known as IP, for Ingress Protection, is used. This can be converted to an equivalent NEMA rating using TABLE 2.
The IP Code consists of the letters I and P followed by two digits or one digit and one letter and an optional letter. As defined in international standard IEC 60529, IP Code classifies and rates the degrees of protection provided against the intrusion of solid objects (including body parts like hands and fingers), dust, accidental contact, and water in mechanical casings and with electrical enclosures.
The standard aims to provide users with more detailed information than vague marketing terms such as "waterproof." However, no edition of the standard is openly published for unlicensed readers, hence leaving room for varying interpretation.
The digits (IP numerals) indicate conformity with the conditions summarized in the tables below. Where there is no protection rating with regard to one of the criteria, the digit is replaced with the letter X.
For example, an electrical socket rated IP22 is protected against insertion of fingers and will not be damaged or become unsafe during a specified test in which it is exposed to dripping water. IP22 or 2X are typical minimum requirements for the design of electrical accessories for indoor use.
TABLE 3 IP Code for Solids (First Digit)
===
Effective Against
No protection against contact and ingress of objects
Any large surface of the body, such as the back of a hand, but no protection against deliberate contact with a body part
Fingers or similar objects
Tools, thick wires, etc.
Most wires, screws, etc.
Ingress of dust is not entirely prevented, but it must not enter in sufficient quantity to interfere with the satisfactory operation of the equipment; complete protection against contact
No ingress of dust; complete protection against contact
===
Solids, First Digit
The first digit indicates the level of protection that the enclosure provides against access to hazardous parts (for example, electrical conductors, moving parts) and the ingress of solid foreign objects, as shown in TABLE 3.
Liquids, Second Digit
The second digit categorizes the protection of equipment inside the enclosure against harmful ingress of water, as shown in TABLE 4.
TABLE 4 IP Code for Liquids (Second Digit) (Continued)
TABLE 5 IP Codes for Access to Hazardous Parts
Additional Letters
The standard defines additional letters that can be appended to classify only the level of protection against access to hazardous parts by persons, as shown in TABLE 5.
Additional letters can be appended to provide further information related to the protection of the device, as illustrated in TABLE 6.
TABLE 7 IK Impact Resistance Codes
TABLE 6 IP Codes Describing Test Conditions Mechanical Impact Resistance
An additional number has sometimes been used to specify the resistance of equipment to mechanical impact. This mechanical impact is identified by the energy needed to qualify a specified resistance level, which is measured in joules (J). The separate IK number specified in EN 62262 has now superseded this measurement.
Although dropped from the third edition of IEC 60529 onward- and not present in the EN version-older enclosure specifications will sometimes be seen with an optional third IP digit denoting impact resistance. Newer products are likely to be given an IK rating instead. There is not an exact correspondence of values between the old and new standards. These codes are shown in TABLE 7.
