Gear thread

MEASURMENTS AND TESTING OF GEARS

Terminology of gear tooth

Gear tooth mainly used for transmission of power and motion.



Terminology of gear tooth1. Addendum2. Dedundum3. Tooth Thickness4. Pitch5. Top land6. Bottom Land7. Face Width8. Circular Pitch9. Pitch Circle10. Base Radius.


Base Circle. It is the circle from which involute form is generated. Only the base circle on a gear is fixed and unalterable.


Base Circle. It is the circle from which involute form is generated. Only the base circle on a gear is fixed and unalterable.

DO-outside diameter, D- pitch diameter, DB- base diameter, DR- root diameter,

For involute teeth


Pitch Circle Diameter (P.C.D.). It is the diameter of a circle which by pure rolling action would produce the same motion as the toothed gear wheel. This is the most important diameter in gears.


Module. It is defined as the length of the pitch circle diameter per tooth. Thus if P.C.D. of gear be D and number of teeth N, thenmodule (m)=D N . It is generally expressed in mm.∕

Diametral Pitch. It is expressed as the number of teeth per inch of the P.C.D. D.P.=D N , D expressed in inch.∕


Circular Pitch (C.P.). It is the arc distance measured around the pitch circle from the flank of one tooth to a similar flank in the next tooth..’. C.P. = ∏D N=∕ ∏m


Addendum. This is the radial distance from the pitch circle to the tip of the tooth. Its value is equal to one module.


Clearance. This is the radial distance from the tip of a tooth to the bottom of a mating tooth space when the teeth are symmetrically engaged. Its standard value is 0.157 m.

Dedendum. This is the radial distance from the pitch circle to the bottom of the tooth space.Dedendum=Addendum+ Clearance=m+0.157 m=l.157 m.


Blank Diameter. This is the diameter of the blank from which gear is a t. It is equal to P.C.D. plus twice the addenda.Blank diameter =P.C.D.+2m.=mN+2m = m(N+2).

Terminology of gear tooth Tooth Thickness. This is the arc distance measured along the pitch circle from its intercept with one flank to its intercept with t le other flank of the same tooth.Normally tooth thickness.=½ C.P.=∏m 2∕


Face of Tooth. It is that part of the tooth surface which is above the pitch surface.Flank of the Tooth. It is that part of the tooth surface which is lying below the pitch surface.

Terminology of gear toothline of action. The teeth of a pair of gears in mesh, contact each other along the common tangent to their base circles, this path is referred to as line of action.

Terminology of gear toothPressure angle ø. The angle between the line of action and the common tangent to the pitch circles is known as pressure angle ø.Db=P.C.D × cosø

Db=base circle dia

Terminology of gear toothPressure angle ø. The angle between the line of action and the common tangent to the pitch circles is known as pressure angle ø.Db=P.C.D × cosø

Db=base circle dia

Terminology of gear toothBase Pitch. It is the distance measured around the base circle from the origin of the involute on the tooth to the origin of a similar involute on the next tooth.Base Pitch=Base Circumference/ No. of teeth=∏×Db/N=∏×Dp cosø /N=∏mcosø.

Terminology of gear toothInvolute Function. It is found from the fundamental principle of the involute, that it is the locus of the end of a thread (imaginary) unwound from the base circle.

Involute function δ=tan ø—øWhere ø is the pressure angle.

Terminology of gear toothHelix Angle : It is the acute angle between the tangent to the helix and axis of the cylinder on which teeth are cut.

Terminology of gear toothLead Angle : It is the acute angle between the tangent to the helix and plane perpendicular to the axis of cylinder

LEAD ANGLE= 90helix angle

Terminology of gear toothBack Lash : The distance through which a gear can be rotated to bring its nonworking flank in contact with the teeth of mating gear

Anti backlash gear

Terminology of gear toothHelix Angle : It is the acute angle between the tangent to the helix and axis of the cylinder on which teeth are cut.

Sources of Errors in Manufacturing Gears

The gears (gear teeth) are generally made by one of the following two methods :(i) Reproducing method(ii) Generating method,


Generating method, in which the cutting tool (hob forms the profiles of several teeth simultaneously during constant relative motion of the tool and blank.

Sources of Errors in Manufacturing GearsReproducing method, in which the cutting tool is formed involve cutter, which forms the gear teeth profiles by reproducing the shape of the cutter itself.In this method, each tooth space is cut independently of the other tooth spaces,


The various sources of errors in the gear made by reproducing method can be due to

(i)incorrect profile on the cutting tool, (ii) incorrect positioning of the tool in relation to the work and (iii) incorrect indexing of the blank.


The sources of error when gears are made by generating method are :

(i) Errors in the manufacture of the cutting tool (ii) errors in positioning the tool in relation to the work and (iii) errors in the relative motion Of the tool and blank during the generating operation.

Measurement of Individual elements

Measurement of tooth thicknessThe permissible error or the tolerance on thickness of tooth is the variation of actual thickness of tooth from its theoretical value. The tooth thickness is generally measured at pitch circle and is therefore, the pitch line thickness of tooth. There are various methods of measuring the gear tooth thickness.(i) Measurement of tooth thickness by gear tooth vernier calliper.(ii) Constant chord method.(iii) Base tangent method.(iv) Measurement by dimension over pins.Gear Tooth Caliper. w=Nm. sin( 90 N) ∕d= = Nm 2[1+2 N− cos(90 N)] ∕ ∕ ∕


The Constant Chord Method.c=constant chord=2A C=∏/ 2 m cos2 ø

d=addendum—PC=m—∏/ 4 m cos ø sin ø=m(1 ∏/ 4 cos ø sin ø )


Base Pitch Measuring Instrument. This instrument has three tips. One is the~!TX53 measuring tip, other one is the sensitive tip whose position can be adjusted by a screw and the further movement of it is transmitted through a leverage system to the dial indicator; and the third tip is the supplementary adjustable stop which is meant for the stability of the instrument and its position cart also be adjusted by a screw. The distance betweenthe fixed and sensitive tip is set to be equivalent to the base pitch of the gear with the help of slip gauges. The properly setup instrument is applied to the gear so that all the three tips contact the tooth profile. The reading on dial indicator is the error in the base pitch.


The Base Tangent Method. (‘David Brown’ tangent comparator). In this method, the span of a convenient number of teeth is measured with the help of the tangent comparator. This uses a single vernier calliperBD=Nm cos ø [tanø –ø −∏ 2N +∏ S N ]∕ ∕


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