Threads and Fasteners

Threads-

Terminology in threads-

External thread-

Internal threads

Different Types of Thread profiles:-Different Types of Thread profiles:-

1 Square Threads1. Square Threads.2. V-threads.3. ACME Threads4 Buttress Threads4. Buttress Threads.5. Whitworth threads

Thread Conventions

Square Threads

V threadsV- threads

ACME threads

Buttress threads

Whitworth Threads

STANDARD REPRESENTATION OF THREADSSTANDARD REPRESENTATION OF THREADS

Visible screw threads - For visiblescrew threads, the crests of threads should be defined by a continuous thick line (type A), and the roots of threads by a continuous thin line (type B).

Hidden screw threads- For hidden screw threads, the crests and the roots should be defined by dashed lines.ySections of threaded parts – For threaded parts shown in section, hatching should be extended to the line defining the crest of the thread.

End view of screw threads - On an end view of a visible screw thread, the thread t h ld b t d b ti f i l d ith ti thiroots should be represented by a portion of a circle, drawn with a continuous thin

line (type B), of length approximately three-quarters of the circumference.

The above conventions apply to assemblies of threaded parts. However, externally threaded parts should always be shown covering internally threaded parts and should not hidden by themthreaded parts and should not hidden by them

METRIC THREAD STANDARDS

ANSI standardg – external threadH – internal thread

e.g., 4H6H : for nut threadg

4g6g – Tolerance Class Designation4g for pitch diameter, 6g for major dia.

Tolerance Grade: 3 (tight fit) to 9 (loose fit) 6: for general applicationsI di ti f t l d i ki d ifIndication of tolerance grade is skipped if both are of grade 6.

ISO Standard, F t l th d (b lt t )For external threads (bolts, etc.):e – Large allowanceg – Small allowance h – No allowance

For internal threads (nuts, etc.):G – Small allowance H – No allowance

Coarse pitch: Fasteners, general application, Fine pitch: for greater strength/vibration resistance

UNIFIED THREAD STANDARDS

UN UnifiedUN – UnifiedC- CoarseF – FineEF – Extra fine threadsEF – Extra fine threads

Unified External Thread Representation

Different Class of FIT1 – loose fit (for quick assbly)2 – normal/regular fit (commonly used)3 – Tight fit with no/marginal clearance

Unified Internal Thread Representation

Representation of ACME Thread

ACME threads are of two broad categories: G (general) and C (Centralizing), the tolerance class varies from 2G (nominal clearance for general applications) to 5G (minimum clearance, for special applications) G (general class of thread) is replaced by C for centralising threads that have critical clearance fits for special applications. Fit for centralizing threads fit range from 2C to 6C

• LH additional indicator in denoting thread means Left Hand thread; Right hand is ; gconsidered default and hence not recordedrecorded.

• SQ: for square thread,

Pipe threadsp

NPT: American National Standard Taper Pipe Threads, conforms to ANSI B 1.20.1NPTF: Dryseal USA Standard Taper Pipe Thread

NPTF: Threads with min clearance/slight interference for tight fitDesigned for sealing the joint without sealing compoundApplication: Hydraulic, Automotive, Refrigeration systems

Different Types of Nuts and Boltsyp

Hexagonal headed bolt

S h d d b ltSquare headed boltGenerally used when the head is to be accommodated in recess. Recess is alsomade up of square shape to prevent rotation of bolt.

Square neck to avoid rotation ofSquare neck to avoid rotation ofBolt while tightening a nut.

Cylindrical or cheese headed bolt

Used when-•Space for accommodating bolt headis less.is less.•Use of spanner is to be avoided.

Rotation of bolt is prevented by p ymeans of a pin

T- headed bolt

P i il d i hi t l t bl f l i•Primarily used in machine tool tables, for clampingfixtures, workpieces, etc.•Fits in T slot provided on them.

Counter sunk headed bolt.

•Used in application where head ofbolt must not project above the surfaceof connected piece.of connected piece.•It may provided with the neck of square cross-section.

Hook Bolt

•This bolt passes through a hole of one pieceonly. other piece is gripped by the hook shapedbolt head.•This is used when it is not possible to drill holein the piece adjoining bolt head.

Eye Bolt-

•Bolt has circular ring as its head.•Used as an appliance to lift heavymachinesmachines.

Stud-

It consists of a cylindrical shank threadedat both sides.The nut end N is threaded for a length slightlyMore than thickness of nut.Metal end M is threaded to length at least equalTo diameter of stud.

Fitting of stud

Washer

Approx Std. DimensionsHexagonal nut

T=D; W=1.5D+3mm; R=1.4D

Rough Rule Dimensions

T=D; R=1 5D

Drawing a hexagonal nut

T=D; R=1.5D

Distance across diagonally opposite corners: 2D

Nuts

Tommy Bar / PinBar / Pin spanner

Locking arrangements for nuts

Due to vibrations, there is always a tendency to for nuts to become slack.

LOCKING SEQUENCE: Th t A i fi t d b lt ti htl ibl•The nut A is first screwed on bolt as tightly as possible.

•The nut B is then screwed on the bolt till it touches the top of nut A.•The nut B is held by spanner and nut A is turned backwards through few degrees.The two nuts are hence locked tightly due to wedging action against each•The two nuts are hence locked tightly due to wedging action against eachother and bolt.

•Nut B carries all the load, hence it is made thicker than nut A. Generally nut A thickness is kept 0 8 times of thickness of nut Bthickness is kept 0.8 times of thickness of nut B

Spring washerSpring washerPartly does the similar job as lock nut

Split pin for locking the nut in place p p g pand prevent its unscrewing

Other means of locking the nut in placethe nut in place

Foundation bolts

Molten Lead /Lead / sulphur

Foundation bolts

Bolt type Picture ApplicationBolt type Picture Application

Square headed Bearings of shaftSquare headed Bearings of shaft

Cylindrical headed Big end of connecting rods, Cylindrical headed eccentrics, etc.

T-headedMachine tool tables, boiler mountings such as stopT headed mountings such as stop

valve, feed check valve etc.

Counter-sunk headed Where the bolt head must Counter sunk headed not project above surface

Eye bolt For lifting heavy machinesEye bolt For lifting heavy machines

Connect cylinder cover to engine cylinder

Studengine cylinder

Situations where there is no space for bolt heads