MECH 313 Engineering Drawing & Design Lecture 7

Transcription

1 MECH 313 Engineering Drawing & Design Lecture 7

2 Outline Keys, Splines and Serrations Pin Fasteners Retaining Rings Springs Rivets Welded Fasteners Adhesive Fastening

3 Keys Splines and Serrations The figure shows other fasteners than the threaded fasteners termed as miscellaneous type

4 Keys Splines and Serrations A key is a piece of steel lying partly in a groove in the shaft and extending into another groove in the hub The groove in the shaft is referred to as a keyseat, and the groove in the hub or surrounding part is referred to as a keyway A key is used to secure gears, pulleys, cranks, handles, and similar machine parts to shafts, so that the motion of the part is transmitted to the shaft, or the motion of the shaft to the part, without slippage

5 Keys Splines and Serrations The most common types are shown in Fig Square and flat keys Widely used in industry The width of the square and flat key should be approximately one-quarter the shaft diameter These keys are also known as squared- taper or flat-tapered dkeys Proper key selection, refer Table 21 in the Appendix Gib Head Key Similar to square and flat key This has got a head for easy removal

6 Keys, Splines and Serrations The Pratt and Whitney key Is rectangular with rounded ends Two-thirds of this key sits in the shaft; one- third sits in the hub Woodruff keys Semicircular key fits in semicircular keyseat in the shaft and rectangular keyway in the hub Width of the key is 1/4 th the diameter of the shaft Diameter of this key is shaft diameter Half the key sits in the shaft and half the key fits into the hub The last 2 digits give the in 1/8 th of an in and the digits before last 2 give the width in 1/32 of an in. So the key here is 1210 that is 12/32 X 10/8 in So the key here is 1210 that is 12/32 X 10/8 in. or 3/8 x 1¼ inch

7 Keys, Splines and Serrations Dimensioning of Keyseats keyseats and keyways are dimensioned by width, depth, location, if needed d by length The depth is dimensioned from the opposite side of the shaft or hole Tapered Keyseats The depth of tapered keyways in hubs, which is shown on the drawing, is the nominal depth H/2 minus an allowance. This is always the depth at the large end of the tapered keyseat and is indicated on the drawing by the abbreviation LE (Large End).

8 Keys, Splines and Serrations Splines and Serrations A splined shaft is a shaft having multiple grooves, or keyseats, cut around its circumference for a portion of its length, in order that a sliding engagement may be made with corresponding internal grooves of a mating part Splines are capable of carrying heavier loads than keys, permit lateral movement of a part while maintaining positive rotation, and allow the attached part to be indexed or changed to another angular position. Fig Straight Side Splines

9 Keys, Splines and Serrations Involute Splines - Similar in shape to involute gear teeth but have pressure angles of 30, 37.5, or 45 There are two types of fits, the side fit and the major diameter fit

10 Keys, Splines and Serrations Straight-Side Splines most popular and used in automobile industries (11-1-6) Serrations are shallow involute splines with 45 angles, used for holding parts such as knobs on steel shafts

11 Keys, Splines and Serrations Drawing Data It is essential that a uniform system of drawing and specifying splines and serrations be used on drawings Symbol indicating the type of spline, type of fit, pitch dia, No of teeth and pitch for involute splines and outside dia for straight sided teeth

12 Pin Fasteners Inexpensive and effective method of assembly when the loading is primarily Shear Semi Permanent require pressure for installation ti and removal. The key design rules are Avoid conditions in which the direction of vibration is parallel to pin axis Keep the shear plane of pin a min distance of one from end of the pin If engaged length is minimum and appearance unimportant pins can protrude the length at each end for increase locking Machine Pins 4 types are considered as important

13 Pin Fasteners Machine Pins for proper size selection refer figure Radial Locking Pins there are two basic pin forms. They are: Solid with grooved faces Hollow spring pins They can either be slotted or spiral wrapped

14 Pin Fasteners Grooved Straight Pins Locking is provided by parallel longitudinal grooves uniformly spaced around surface Rolled or pressed into solid stock, the grooves expand the effective diameter

15 Pin Fasteners Grooved Straight Pins When the pin is driven into a drilled hole slightly larger than the nominal pin diameter, elastic deformation of the raised groove edges produces a secure force fit with the hole wall Locking force developed by a groove-pin assembly is a function of pin and effective length of engagement

16 Pin Fasteners Grooved Straight Pins - Applications

17 Pin Fasteners Hollow Spring Pins Use the resilience of hollow cylinder walls to hold in place The two main types are spiralwrapped and slotted tubular pins Both pin forms are made to controlled diameters greater than the holes into which they are pressed. Compressed when driven into the hole, the pins exert spring pressure against the hole wall along their entire engaged length to develop locking action Locking force of a spiral-wrapped pin is a function of length of engagement, pin diameter, and wall thickness. Pins are available for light, medium, and heavy-duty applications Slotted tubular pins come in standard sizes. Readily adaptable to Slotted tubular pins come in standard sizes. Readily adaptable to manual assembly techniques, these pins offer a tough, resilient, selflocking fastener that can withstand high shock and vibration loads

18 Pin Fasteners Hollow Spring Pins Spiral-wrapped pins have an advantage over slotted pins in automatic assembly because they cannot interlock during feeding They may also resist vibration and absorb shock better than slotted pins because the coil design can flex after assembly, while the slotted version cannot flex after the gap is closed For maximum shear strength, the pin should be assembled so that the gap is in line with the direction of load and 180 away from the point of application. The maximum shear strength value provided by this orientation g p y represents an increase of about 6% over the minimum value

19 Pin Fasteners Hollow Spring Pins - Applications

20 Pin Fasteners Quick Release Pins Mainly used for rapid manual assembly and disassembly, quick-release pins use a mechanism to provide a locking action. They use a clearance fit in holes formed to nominal diameters and are divided into two major types - push-pull pins and positive-locking pins Push-pull pins - are made with a solid or a hollow shank containing a detent assembly in the form of a locking lug, button, or ball which is backed up by a resilient core, plug, or spring. Primary function of these pins is to fasten parts under shear loading and not for loads in tension Positive-locking pins - have a locking action that is usually independent of insertion and removal forces. These pins are also primarily suited for shear-load applications, though some tension loading can be tolerated without affecting pin function

21 Pin Fasteners Quick Release Pins Applications

22 Retaining Rings Retaining Rings Retaining or Snap rings used to provide removable shoulder to accurately locate, retain or lock components on shaft and in bores of housings They are easily installed and removed as they are made of spring steel They have high shear strength and impact capacity It can also be used for correction of end play caused by accumulation of tolerances

23 Retaining Rings Retaining Rings Wire-Formed Retaining Rings - The wire-formed retaining ring is a split ring formed and cut from spring wire of uniform crosssectional size and shape. The wire is cold-drawn or rolled into shape from a continuous coil or bar. Then the gap ends are cut into various configurations for ease of application and removal Spiral-Wound Retaining Rings - Spiral-,wound retaining rings consist of two or more turns of rectangular material, wound on edge to provide continuous crimped or uncrimped coil. Stamped Retaining Rings unlike wire-formed rings they have tapered radial dia that decreases symmetrically from the center section to the free ends. This permits the rings to remain circular when expanded for assembly over a shaft or contracted for insertion into housing. This ensures maximum contact surface with the bottom of the groove

24 Retaining Rings Stamped Retaining Rings Common Types

25 Springs Classified into 3 general groups Controlled action Springs have well defined function and constant range of action. Used in Valves and die Variable action Springs have changing ranges due to various conditions. Used in Suspension, clutch and cushion springs Static Springs exert constant pressure or tension between parts. Used in packing or bearing pressure, antirattle and seal springs The figure shows common springs in use

26 Springs Types of Springs Type of spring is determined by the characteristics such as function, shape of material, application or design Spring nomenclature, important parameters shown in

27 Springs Types of Springs Compression Springs open coiled helical spring that offers resistance to compressive force Common end styles are shown in figure a Plain open ends are produced by straight cutoff with no reduction in helix angle (11-4-4) Ground open ends are produced by parallel grinding of open-end coil springs. Advantages is improved stability and larger number of coils Plain closed ends are produced with straight cutoff but with reduction in helix angle to get closed end coils. (more stable) Ground closed ends are produced by parallel grinding of open-end coil springs maximum stability a

28 Springs Types of Springs Extension Springs are closed coiled helical spring that offers resistance to pulling force Common end styles are shown in figure b The end of the extension spring is the most stressed part Proper consideration should be given for selection of end style Different types of ends can be used on the same type of spring b

29 Springs Types of Springs Torsion Springs exerts pressure along a path that is a circular arc, i.e. providing torque or torsion (example motor springs, power springs etc.) It is a helical springs of round, square, or rectangular wire (strength). Few of the most common end styles are shown in figure c The end of the extension spring is the most stressed part Proper consideration should be given for selection of end style Different types of ends can be used in same spring c

30 Springs Flat Springs Made of flat material in such a way to apply force in desired direction when deflected in opposite direction (eg leaf spring and Belleville spring) Leaf Spring composed of series of flat springs together and arranged to provide approximately uniform stress distribution along its length They are used in multiple arrangements a Belleville Spring are washer shaped mad in form of short truncated cone They are assembled in series to accommodate greater deflection; in parallel to resist greater forces; or a combination of both Arrangements shown in b

31 Springs Spring Drawings Schematic drawing of helical spring is used to save time in working drawings (11-4-6) As in screw thread representation, ti straight lines are used instead of helical curves On assembly drawings springs are shown in section (11-4-7) Use of solid black shading or crosshatching lines are used This is decided based on the This is decided based on the size of wire diameter of the spring

32 Springs Spring Dimensioning Following information is given while dimensioning springs in drawing ( ) Size, shape and material used in spring Dia (inside or outside) Pitch or No of coils Shape of ends Length Load and rate If schematic is used, of the wire to be stated Example ONE HELICAL TENSION SPRING 3.00 LG (OR NUMBER OF COILS), 0.5 ID, PITCH.25, 18 B & S GA SPRING BRASS WIRE

33 Springs Spring Clips Typical spring clips are self-retaining, one-piece fasteners that slip into a mounting hole or onto a flange or panel edge Secondary fastening devices such as rivets, studs, or screws are not needed because the clips are held by spring tension and do not loosen easily through vibration. The clips also compensate for tolerance buildup and misalignment They are commonly used to join two panel surfaces, as in refrigerator door liners. Other configurations are used to fasten cables, molding trim, gaskets, and fabrics They may be installed with finger pressure for light duty or with power tools for heavy applications Various configurations and applications shown in

34 Springs Spring Clips Applications

35 Rivets Standard Rivets Riveting is a popular method of joining due to its simplicity, dependability d and low cost Rivets are permanent fasteners (unlike bolts and screws) A rivet is a ductile metal pin that is inserted through holes in two or more parts, and having the ends formed over to securely hold the parts Rivets can be used to join dissimilar materials of various thicknesses Can serve as fasteners, pivot shafts, spacers or inserts Can be used to fasten finished parts (after plating painting etc) If for maintenance, they have to be knocked out and clinched with new rivets once again for assembly They are neither watertight or airtight. But at higher cost they can be made to with addition of sealing compound

36 Rivets Type of Riveted Joints

37 Rivets Large Rivets Used in structural work (buildings, bridges etc) Rivets have been replaced by high strength bolts due to strength, cost and most importantly noise factor Rivet joints are either Butt or Lap joints as seen before Common types of large rivets are shown in

38 Rivets Large Rivets When shop rivets (which are put in the structure at the shop) are drawn, the diameter of the rivet head is shown on the drawings. For field rivets, the shaft diameter is used Figure shows the conventional rivet symbols adopted by the American and Canadian Institutes of Steel Construction

39 Rivets Rivets for Aerospace Equipment + shows the position of the rivet Upper left quad used show part number Upper right show preformed head details Lower left to show dimpling or counter sinking (if angle not given it is 100 ) Lower right quad is left free

40 Rivets Symbolic Representation of a Line of Rivets The crosses are aligned along the axes of drawing and number of places for rives indicated. If supplemental info cannot be indicated on the drawing, use of leader lines to indicate them is fine When the rivets are aligned, identical, and equidistant, the symbols should be shown in the first and last positions, together with the total number of pitches and distance (Fig B)

41 Rivets Small Rivets Design of small rivet assemblies is influenced by two major considerations: 1. The joint itself, its strength, appearance, and configuration 2 The final riveting operation in terms of equipment capabilities 2. The final riveting operation, in terms of equipment capabilities and production sequence

42 Rivets Types of Small Rivets Semitubular most widely used. The depth of the hole in rivet measured along the wall does not exceed 112% of the shank dia. The hole may be extruded or drilled (straight or tapered) Full Tubular has a drilled shank with hole depth is more than 112% of the shank dia. This can punch its own hole on fabric and soft materials like plastic, eliminating i punching Bifurcated (Split) the body is sawed or punched to produce pronged shank that can punch its own hole on fabric wood or plastic Compression comprises of two members, the solid or blank rivet and a deep drilled tubular member. They form a interference fit when pressed together

43 Rivets Design Recommendations Rivet Diameters - The optimum rivet diameter is determined, not by performance requirements, but by economics-the costs of the rivet and the labor to install it. The rivet length to-diameter ratio should not exceed 6:1 Rivet Positioning - The location of the rivet in the assembled product influences both joint strength and clinching requirements. The important dimensions are edge distance and pitch distance Edge distance - Is the interval between the edge of the part and the center line of the rivet. The recommended edge distance for plastic materials, either solid or laminated, is between two and three diameters ( depending on the thickness and inherent strength of the material) Pitch distance - Unnecessarily high stress concentrations in the riveted material and buckling at adjacent empty holes can result if the pitch distance is less than three times the diameter of fthe largest rivet in the assembly (metal parts) or five times the diameter (plastic parts)

44 Rivets Design Recommendations

45 Rivets Blind Rivets Used when reverse side of the joint is not accessible. Sometimes they can be used dif other side is accessible as well Classified according to the methods to which they are set. Mainly pull mandrel, drive pin and chemically expanded

46 Rivets Design Considerations Type of Rivet Selection depends on speed of assembly, clamping capacity, available sizes, ease of removal, cost etc Joint Design Allowable tolerance of rivet length vs. assembly thickness hole clearance, joint configuration and type of loading Speed of installation fastest installation is done with power tools. Manual tools specially designed for these can be used without much training In-place costs Blind rivers have lower in-place costs than solid rivets or tapping screws Loading Blind rivet joint is usually in compression than in shear Material thickness some rivets can be set in materials as thin as 0.5mm. If one component is of compressible material (soft) extra large head diameter should be used Edge distance Average edge distance is twice the rivet dia.???

47 Rivets Design Considerations Spacing the pitch should be three times the rivet diameter Length needed for clinching action varies. Rivet manufacturers provide the grip ranges for their rivets Backup Clearance full entry is essential for proper clinch. Sufficient backup clearance must be provided to accommodate the full length of the unclinched rivet Blind Holes or Slots useful application of blind rivet is in fastening members in blind hole. In figure , B and C are stronger than A as there is no provision for formed head in A Riveted Joints lap pj joint (B) must have sufficient material beyond the hole for strength. Excessive material can cause vibration (C) Flush Joints are made by countersinking one of the section and using a rivet with countersunk head

48 Rivets Design Considerations

49 Rivets Design Considerations Weatherproof Joints rivet can be sealed by capping it A; by plugging it B; or using cap and plug C. for obtaining better sealing mastic or gasket is used between the sections Rubber Plastic and Fabric Joints some plastics such as reinforced molded fiberglass which are reasonably rigid do not pose problems for small rivets. But if the material is fabric or it is soft and flexible use methods shown in A or B. if it is not possible use a backup strip as shown in C Pivoted Joints 3 of the many ways of pivoted joints are shown Attaching a Solid Rod or Tubing when a rod is attached to other members generally the rivet completely l passes through h the rod or tubing joint as shown for structural assemblies Using pull-up by properly positioning rivets unlike parts can be pulled together using the riveting force Honeycomb Sections inserts should be employed to strengthen

50 Rivets Design Considerations

51 Welded Fasteners The most common welded fasteners are screws and nuts. Here they are grouped into Resistance welded fasteners and arc- welded studs Resistance-Welded Fasteners An externally or internally threaded metal part designed to be fused permanently in place by standard production welding equipment. Methods of resistance welding: Projection welding Spot welding

52 Welded Fasteners Resistance-Welded Fasteners Projection welding - Heat is localized through embossed or coined projections on the fastener. During the welding process, the projections coalesce with the part surface to form the weld. For best results, a press-type welder with electronic controls is usually recommended. This type of welder gives positive electrode alignment and equalized welding pressures Spot welding - The current is directed through the entire area under the electrode tip. Welding is usually performed by a spot welder. This type of welding equipment can satisfactorily weld a number of fastener designs Comparison - Spot welding costs less than projection welding. However, the projection welder is more flexible and permits far greater latitude in design

53 Welded Fasteners Design Considerations Before fasteners can be used, three basic requirements must be met (Fig , Fig , Fig ) 1. The materials to be joined, both part and fastener, must be suitable for resistance welding 2. The parts to be welded must tbe portable enough to be carried to the welder 3. Production volume should be great enough to justify tooling costs

54 Welded Fasteners Design Considerations

55 Welded Fasteners Design Considerations

56 Welded Fasteners Arc-Welded d Studs Two basic stud welding processes; electric arc and capacitor discharge Electric Arc stud welding most widely used is semiautomatic electric arc process. To avoid burnthrough, the plate thickness should be at least one-fifth of the welding base diameter Capacitor discharge stud welding this welding process derives heat from an arc produced by a rapid discharge of stored electrical energy

57 Welded Fasteners Design Considerations In most cases the stud welding process is determined by the fastener and the base metal size Capacitor Discharge (CD) stud welding is generally used to weld smaller diameter fasteners to thin base metals Arc stud welding is generally used to weld large diameter fasteners to rougher and thicker base metals

58 Adhesive Fastening To allow greater flexibility in design, styling and materials; and cost reduction adhesive fastening is used. As with other methods these have their own advantages and disadvantages. For selection table 51 in appendix gives the application data Adhesion Versus Stress Adhesion holds materials together th and stress pulls them apart Tensile pull equally over entire joint. All adhesive contribute to bond strength Shear direction across the bond. Bond material slide over one another Peel one of the surfaces is flexible and stress is concentrated on the thin line at the edge of the bond Cleave concentrated on one edge and the other edge has zero stress

59 Adhesive Fastening Advantages Resistance to stress is the reason for using adhesive fastening Adhesives allow uniform distribution of stress over the entire bond area 1. (Fig ). 1) This eliminates stress concentration caused by rivets, bolts, spot welds, and similar fastening techniques. Lighter, thinner materials can be used without sacrificing strength. 2. Adhesives can effectively bond dissimilar materials. 3. Continuous contact between mating surfaces effectively bonds and seals against many environmental conditions. 4. Adhesives eliminate holes needed for mechanical fasteners and surface marks resulting from spot welding, brazing

60 Adhesive Fastening Limitations 1. Adhesive bonding can be slow or require critical processing. This is particularly true in mass production. Some adhesives require heat and pressure or special jigs and fixtures to establish the bond 2. Adhesives are sensitive to surface conditions. Special surface preparation may be required 3. Some adhesive solvents present hazards. Special ventilation may be required to protect employees from toxic vapors 4. Environmental conditions can reduce bond strength of some adhesives. Some do not hold well when exposed to low temperatures, high humidity, severe heat, chemicals, water

61 Adhesive Fastening Joint Design Joints need to be specifically designed All the bonded area share the load equally Joint configuration should be designed so that basic stress is primarily in shear or tensile Cleavage and peel to be minimized or eliminated

62 Adhesive Fastening Joint Design Lap Joints practical and applicable in bonding thin materials Simple lap joint can fail by cleavage and peel stress when thin materials are used Tapered single lap joint is better than simple lap Tapered edge allows bending on edges but Joggle gives more uniform stress distribution Double butt lap joint gives better load distribution than the others This type of joint requires machining which is not always possible in thinner gage materials Double scarf lap joints have better resistance to bending forces than double butt joints, again machining is required

63 Adhesive Fastening Joint Design Angle Joints give rise to peel or cleavage stress depending on the gage of metal. To reduce cleavage, typical approaches are shown Butt Joints illustrated recessed butt joints are recommended d Cylindrical Joints the T joint and overlap slip joint are typical for bonding cylindrical i l parts such as tubing, bushings and shafts

64 Adhesive Fastening Joint Design Corner Joints sheet metal assembled with adhesives using supplementary attachments. This permits joining and sealing in one operation. Typical joints are Corner Joints rigid members as in decorative fames can be adhesively bonded. End lap pjoints are simplest, they require machining. Mortise and tenon joints are excellent from design point, and require machining. Mittered joints are good if both members are hollow extrusions Stiffener Joints deflection and flutter of thin metal sheets can be minimized with adhesive bonded stiffeners

65 Adhesive Fastening Joint Design