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Joints types

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.

Axis Pitch

Angle

Crest

Root

Side

8Minorn

Thread Depth

MajornPn

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

General use.

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?


– Then the threads are cut using a tap.

The tap drill hole is longer than the length of the threads. Why?

Incomplete threads


– Chamfers are sometimes cut to allow for easy engagement.

Manufacturing Threads• External Threads

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

Manufacturing Threads• 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.

Schematic Representation• Pros and Cons?

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

Schematic Representation• 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

Unified Threads (inch)• 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).

Unified Threads (inch)• 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).

Unified Threads (inch)• 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

Unified Threads (inch)• 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

Unified Threads (inch)• 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.

Unified Threads (inch)• 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 DIA20 threads per inch (P = 1/20 = .05)Thread form & series – UN Coarse

Thread Class – Normal Production

External ThreadsRight 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)


(See Appendix B)

10 – 32 UNF


(See Appendix B)– Is the major diameter 10 inches? No

10 – 32 UNF


(See Appendix B)– Is the major diameter 10 inches? 0.190

10 – 32 UNF


(See Appendix B)– What is the minor diameter?

10 – 32 UNF


(See Appendix B)– What is the minor diameter?

10 – 32 UNFD – 1.0825P = 0.190 – 1.0825/32 = 0.156

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)

21.5

1414.5

13.614.2

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.

Bolt and Screw Clearances• 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.

Bolt and Screw Clearances• 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.

Bolt and Screw Clearances• 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)

Bolt and Screw Clearances• 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

9/3213/16

Exercise 5-6• A 5/16 - 18 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


Max. Head diameter A = 1.5(5/16)=0.469

Max. Height of head H = D = 5/16


C’Bore dia.

C’Bore depth


Max. Head diameter A = 1.5(5/16)=.469



C’Bore dia.

C’Bore depth




Normal clearance hole dia. C = D + 1/32 = 11/32

C’Bore dia. B = 17/32

C’Bore depth




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.


Screw

Major dia. 8

Head dia.

C’Sink dia.



Screw

Major dia. 8

Head dia. A = 17.92

C’Sink dia.



Screw

Major dia. 8

Head dia. A = 17.92

C’Sink dia. Y = 17.92


Or, Y = A + 3 = 20


Screw

Major dia. 8

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 keysThese 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 keysThese 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

Key with Gib headKey with square or rectangular head

W = 0.25 D + 2 mmT = 0.67 W (at the thicker end)

taper = 1:100H = 1.75 T

B = 1.5 T

parallel or feather keyA 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

SplinesSplines 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 keysIt 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 + 0.1 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 pinLocking 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 inwidth, 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 forcesalong their axes. These joints are not suitable where the members are under rotation

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 makethe 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. 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.

joints types jointspermanentsolderedbrazedadhesivedetachablethreaded,screw & nutkeyed &...

Documents

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