JOINTS.

Name: JOINTS.
Uploaded: 2017-07-03T23:19:30+00:00
Duration: PTM32S17
Channel: Tracy Griffith
Description: JOINTS.

JOINTS

Joints types Joints Permanent Soldered Brazed Adhesive Detachable
Threaded ,Screw & Nut Keyed & Splined Cotter and Pins Interference-fit

Threaded-joints This type of connection is made by threaded fastening members such as screws, bolts, studs and nuts. Threaded connections have found extensive application > 60% of all elements Threaded joints can: Can transmit considerable axial force related to wedging action Can be fixed in any position related to considerable friction Are easily adaptable to precision manufacture

Types of threaded joints and fasteners
Bolted connection Are the simplest and cheapest, not require thread in the parts, joints 2 thin parts

Screw joints Are useful where one of the parts to be joined is rather thick , a screw is driven into a threaded holes in the thicker part

Stud joints Are made where one of the parts is much thicker than the other , and the connection is to be frequently disassembled in service.

Set screw connection Are employed to prevent the relative displacement of the parts at the joints

Self taping screw To fasten together soft-metal parts which are not likely to be unfastened in service.

Thread Definitions Screw Thread: A ridge of uniform section in the form of a helix.

Thread Definitions External Thread:
An external thread is cut using a die or a lathe.

Thread Definitions Internal Thread: Internal threads are on the inside of a member. An internal thread is cut using a tap.

Thread Definitions Major DIA (D): The largest diameter (For both internal and external threads). Minor DIA (d): The smallest diameter. Depth of thread: (D-d)/2 Pitch DIA (dP): The diameter at which a line cuts the spaces and threads equally.

Thread Definitions Crest: The top surface. Root: The bottom Surface.
Side: The surface between the crest and root.

Thread Definitions Pitch (P): The distance from a point on a screw thread to a corresponding point on the next thread (in/Threads). Angle of Thread (A): The angle between the threads. Screw Axis: The longitudinal centerline. Lead: The distance a screw thread advances axially in one turn.

Thread Definitions Right Handed Thread: Advances when turned CW. (Threads are assumed RH unless specified otherwise.) Left Handed Thread: Advances when turned CCW.

Identify the Pitch, Screw Axis and Thread Angle.
Crest Pitch Axis Root Thread Depth Minorn Pn Majorn 8 Angle Side

Types of Thread There are many different types of thread forms (shape) available. The most common are; Unified Metric

Types of Thread Thread form choice depends on;
what it will be used for length of engagement load etc…

Types of Thread (Form) Thread Name Figure Uses Unified screw thread
General use. ISO metric screw thread Square Ideal thread for power transmission.

Types of Thread (Form) Thread Name Figure Uses ACME
Stronger than square thread. Buttress Designed to handle heavy forces in one direction. (Truck jack)

Manufacturing Threads
Internal Threads First a tap drill hole is cut with a twist drill. The tap drill hole is a little bigger than the minor diameter. Why?

Internal Threads Then the threads are cut using a tap. Incomplete threads The tap drill hole is longer than the length of the threads. Why?

Internal Threads Chamfers are sometimes cut to allow for easy engagement.

External Threads You start with a shaft the same size as the major diameter.

External Threads The threads are then cut using a die or on a lathe.

Detailed Representation
A detailed representation is a close approximation of the appearance of an actual screw thread.

Detailed Representation
Pros and Cons? Pro: Looks good and clearly represents a thread. Con: Takes a long time to draw.

Schematic Representation
The schematic representation uses staggered lines to represent the thread roots and crests.

Pros and Cons? Pro: Nearly as effective as the detailed representation and easier to draw. Con: Still takes some time to draw.

Rules of use for Schematic threads Should not be used for hidden internal threads or sections of external threads.

Simplified Representation
The simplified representation uses visible and hidden lines to represent the major and minor diameters.

Simplified Representation
Pros and Cons? Pro: Simple and fast to draw. Con: Doesn’t look like a thread.

Simplified Internal Threads

Drawing Screw Threads Thread tables in the appendix can be used to look up value for the; Pitch Minor diameter Tap drill diameter If screw thread tables are not available, the minor diameter can be approximated as 75% of the major diameter.

Unified Threads (inch)
After drawing a thread, we need to identify the size and thread form in a thread note. Thread Note

Unified Thread Note Components

Major Diameter: The largest diameter. Threads per inch: Number of threads per inch for a particular diameter. Equal to one over the pitch (1/P).

Thread Form and Series: The shape of the thread cut. UNC = Unified National coarse. For general use. UNF = Unified National fine. Used when high degree of tightness is required. UNEF = Unified National extra fine. Used when length of engagement is limited (Example: Sheet metal).

Thread Class: Closeness of fit between the two mating threaded parts. 1 = Generous tolerance. For rapid assembly and disassembly. 2 = Normal production 3 = High accuracy

External or Internal Threads A = External threads B = Internal threads Right handed or left handed thread RH = Right handed (right handed threads are assumed if not stated.) LH = Left handed

Depth of thread: The thread depth is given at the end of the thread note and indicates the thread depth for internal threads This is not the tap drill depth.

Thread class is assumed to be 2. Threads are assumed to be RH. May be left off if assumptions hold.

Exercise 5-2 Identify the different components of the following Unified National thread note. 1/4 – 20 UNC – 2A – RH 1/4 20 UNC 2 A RH .25 inch Major DIA 20 threads per inch (P = 1/20 = .05) Thread form & series – UN Coarse Thread Class – Normal Production External Threads Right Handed Threads

Unified National Thread Tables
Standard screw thread tables are available in order to look up the: Major diameter Threads per inch Minor diameter or Tap drill size. Thread tables are located in Appendix B.

Exercise 5-3 Write the thread note for a #10 fine thread. (See Appendix B)

Exercise 5-3 Write the thread note for a #10 fine thread. (See Appendix B) 10 – 32 UNF

Exercise 5-3 Write the thread note for a #10 fine thread. (See Appendix B) Is the major diameter 10 inches? No 10 – 32 UNF

Exercise 5-3 Write the thread note for a #10 fine thread. (See Appendix B) Is the major diameter 10 inches? 0.190 10 – 32 UNF

Exercise 5-3 Write the thread note for a #10 fine thread. (See Appendix B) What is the minor diameter? 10 – 32 UNF

Exercise 5-3 Write the thread note for a #10 fine thread. (See Appendix B) What is the minor diameter? D – P = 0.190 – /32 = 0.156 10 – 32 UNF

Metric Threads The metric thread note can contain a pitch diameter tolerance. What is the pitch diameter? Let’s see.

Pitch Diameter The pitch diameter cuts the threads at a point where the distance of the spaces equal the distance of the threads.

Metric Thread Note Components

Metric Threads Metric Form: Placing an M before the major diameter indicates the metric thread form.

Metric Threads Major Diameter: The largest diameter
Pitch: (P) Millimeters per thread.

Metric Threads Tolerance Class: It describes the looseness or tightness of fit between the internal and external threads. Number = Tolerance grade Letter = Tolerance position

Metric Threads Tolerance Class:
Tolerance Grade: Smaller numbers indicate a tighter fit. Tolerance Position: Specifies the amount of allowance. Upper case letters = internal threads Lower case letters = external threads.

Metric Threads Tolerance Class: Two classes of metric thread fits are generally used. 6H/6g = General purpose 6H/5g6g = Closer fit. A tolerance class of 6H/6g is assumed if it is not specified.

Metric Threads Right handed or Left handed thread:
RH = Right handed (right handed threads are assumed if not stated.) LH = Left handed

Metric Threads Depth of thread: It indicates the thread depth for internal threads, not the tap drill depth.

Metric Thread Note A tolerance class of 6H/6g is assumed.
Threads are assumed to be RH. May be left off if assumptions hold.

Exercise 5-4 Identify the different components of the following metric thread notes. M10 x 1.5 – 4h6h – RH M 10 1.5 4h 6h Int. or Ext. RH Metric Form 10 mm Major DIA Pitch – mm/threads Pitch DIA tolerance Minor DIA tolerance External Right handed threads

Metric Thread Tables Standard screw thread tables are available in order to look up the; Major diameter Pitch Tap drill size or Minor diameter Thread tables are located in Appendix B.

Exercise 5-5 For a n16 internal metric thread, what are the;
two available pitches, the tap drill diameter, and the corresponding minor diameter for the mating external threads.

Find this page.

Exercise 5-5 For a n16 internal metric thread. Pitch Tap drill DIA
Minor DIA (External) 2 14 13.6 1.5 14.5 14.2

Exercise 5-5 For a n16 internal metric thread.
Which has the finer thread? Pitch = 2 Pitch = 1.5

Exercise 5-5 Write the thread note for a 16 mm diameter coarse thread.
M16 x 2

Drawing Bolts D represents the major diameter.
Nuts are drawn in a similar fashion.

Bolt and Screw Clearances
Bolts and screws attach one material with a clearance hole to another material with a threaded hole.

The size of the clearance hole depends on; the major diameter of the fastener and the type of fit normal close loose

Table 5-2 (Normal fit clearances)
Other fits may be found in Appendix B.

Sometimes bolt or screw heads need to be flush with the surface. This can be achieved by using either a counterbore or countersink depending on the fasteners head shape.

Counterbores: Counterbores are holes designed to recess bolt or screw heads below the surface of a part. Typically, CH = H + 1/16 (1.5 mm) and C1 = D1 + 1/8 (3 mm)

Countersink: Countersinks are angled holes that are designed to recess screws with angled heads. Typically, C1 = D1 + 1/8 (3 mm) Appendix B gives other counterbore, countersink and shaft clearance holes.

Exercise 5-6 What is the normal fit clearance hole diameter for the following nominal bolt sizes. Nominal size Clearance hole 1/4 3/4 9/32 13/16

Exercise 5-6 A 5/ UNC – Socket Head Cap Screw needs to go through a piece of metal in order to screw into a plate below. The head of the screw should be flush with the surface.

Exercise 5-6 5/16 - 18 UNC – Socket Head Cap Screw
Fill in the following table. Refer to Appendix B. Head diameter Height of head Normal clearance hole dia. C’Bore dia. C’Bore depth

D = 5/16

Fill in the following table. Refer to Appendix B. Max. Head diameter A = 1.5(5/16)=0.469 Max. Height of head H = D = 5/16 Normal clearance hole dia. C’Bore dia. C’Bore depth

Fill in the following table. Refer to Appendix B. Max. Head diameter A = 1.5(5/16)=.469 Max. Height of head H = D = 5/16 Normal clearance hole dia. C’Bore dia. C’Bore depth

Fill in the following table. Refer to Appendix B. Max. Head diameter A = 1.5(5/16)=.469 Max. Height of head H = D = 5/16 Normal clearance hole dia. C = D + 1/32 = 11/32 C’Bore dia. B = 17/32 C’Bore depth

Fill in the following table. Refer to Appendix B. Max. Head diameter A = 1.5(5/16)=.469 Max. Height of head H = D = 5/16 Normal clearance hole dia. C = D + 1/32 = 11/32 C’Bore dia. B = 17/32 C’Bore depth >H (H+1/16 = 3/8)

Exercise 5-6 An M8x1.25 Flat Countersunk Head Metric Cap Screw needs to go through a piece of metal in order to screw into a plate below. The clearance hole needs to be close and the head needs to be flush with the surface. What should the countersink diameter and clearance hole diameter be?

Exercise 5-6 M8x1.25 Flat Countersunk Head Metric Cap Screw Major dia.
Head dia. C’Sink dia. Close clearance hole dia.

Head dia. A = 17.92 C’Sink dia. Close clearance hole dia.

Head dia. A = 17.92 C’Sink dia. Y = 17.92 Close clearance hole dia. Or, Y = A + 3 = 20

Head dia. A = 17.92 C’Sink dia. Y = 17.92 Close clearance hole dia. 8.4 Or, Y = A + 3 = 20

Keys Keys are machine elements used to prevent relative rotational movement between a shaft and the parts mounted on it, such as pulleys, gears, wheels, couplings, etc.

Keys classifications Keys are classified into: saddle keys sunk keys
round keys

saddle keys These are taper keys, with uniform width but tapering in thickness on the upper side. The magnitude of the taper provided is 1:100. These are made in two forms: hollow saddle key Flat saddle key The two types of saddle keys are suitable for light duty only.

hollow saddle key

Flat saddle key

sunk keys These are the standard forms of keys used in practice, and may be either square or rectangular in cross-section. The end may be squared or rounded. Generally, half the thickness of the key fits into the shaft keyway and the remaining half in the hub keyway. These keys are used for heavy duty

Sunk keys classification
Sunk keys may be classified as: (i) taper keys, (ii) parallel or feather keys (iii) woodruff keys.

taper sunk keys W = 0.25 D + 2 mm T = 0.67 W (at the thicker end)
H = 1.75 T B = 1.5 T Key with square or rectangular head Key with Gib head

parallel or feather key
A parallel or feather key is a sunk key, uniform in width and thickness as well. These keys are used when the parts (gears, clutches, etc.) mounted are required to slide along the shaft; permitting relative axial movement. To achieve this, a clearance fit must exist between the key and the keyway in which it slides, fitted into the keyway provided on the shaft by two or more screws or into the hub of the mounting

Types of parallel or feather key
peg feather key single headed feather key double headed feather key

Splines Splines are keys made integral with the shaft, by cutting equi-spaced grooves of uniform cross-section. The shaft with splines is called a splined shaft. The splines on the shaft, fit into the corresponding recesses in the hub of the mounting, with a sliding fit, providing a positive drive and at the same time permitting the latter to move axially along the shaft

woodruff keys It is a sunk key, in the form of a segment of a circular disc of uniform thickness, As the bottom surface of the key is circular, the keyway in the shaft is in the form of a circular recess to the same curvature as the key. If D is the diameter of the shaft, Thickness of key, W = 0.25 D Diameter of key, d = 3 W Height of key, T = 1.35 W Depth of the keyway in the hub, T1 = 0.5 W mm Depth of keyway in shaft, T2 = 0.85 W

round keys Round keys are of circular cross-section, usually tapered (1:50) along the length. A round key fits in the hole drilled partly in the shaft and partly in the hub . The mean diameter of the pin may be taken as 0.25 D, where D is shaft diameter. Round keys are generally used for light duty, where the loads are not considerable.

How do fasteners lock in place?
Proper torque Lock washers Cotter pins Locking tabs Self locking nuts Thread locking compounds Red LocTite© Blue LocTite© USE the BLUE unless specified. Red will not come off!

Do not re-use Nylon locking nuts Cotter pins
Nuts in critical locations such as connecting rod nuts

Locking devices Locking Nut Locking by split pin
Locking using castle Nut Wile’s lock nut

Locking devices Locking using set screw Grooved Nut
Locking using screw

Locking devices Locking by plate Locking by spring washer

Cotter Pin A cotter is a flat wedge shaped piece, made of steel. It is uniform in thickness but tapering in width, generally on one side; the usual taper being 1:30. The lateral (bearing) edges of the cotter and the bearing slots are generally made semi-circular instead of straight

Cotter joints are used to connect two rods, subjected to tensile or compressive forces along their axes. These joints are not suitable where the members are under rotation

This joint is also used to fasten two circular rods
This joint is also used to fasten two circular rods. In this, the rod ends are modified instead of using a sleeve. One end of the rod is formed into a socket and the other into a spigot

This joint is generally used to connect two rods of square or rectangular cross-section. To make the joint, one end of the rod is formed into a U-fork, into which, the end of the other rod fits in

A knuckle joint is a pin joint used to fasten two circular rods
A knuckle joint is a pin joint used to fasten two circular rods. In this joint, one end of the rod is formed into an eye and the other into a fork (double eye). Knuckle joints are used in suspension links, air brake arrangement of locomotives, etc.

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