WORM and WORM gear :

A worm drive is an arrangement of gears in which a worm ( In the form of screw) meshes with a worm wheel (in the form of spur gear). A worm gear can transmit motion (speed reduction) as well as it can transmit high torque. Another distinguishing feature of worm gear unit is that they transfer motion in 90 degrees.

Worm and worm wheel drive are used for Non-intersecting, Non-Parallel shaft axes at 90 degree gear drive. It is used for high-speed reduction ratios. 

Following are the advantages of worm drive

1. Higher transmission ratio with less number of gears.

2. Comparatively less weight.

3. Compact arrangement, less requirement of space.

4.Noiseless operation

5.Self locking and non-reversible.

A self locking application of worm gear in controlling gate. The position of gate will not change once it is set.

QUESTION AND ANSWERS :

1) State the advantages and drawbacks of Worm gear drives?

Ans :

Following are the advantages of worm and worm gear drive :

1) High speed reduction : This is the most important characteristic of worm gear drives that they can achieve high speed reduction, as high as 1:100. Such large speed reduction is achieved in single pair of worm and worm gear.

2) Compactness : Only single pair of worm and worm gear can obtain very high speed reduction, which would require several gears when spur or helical gears used.

3) Self locking : Worm and worm gear is self locking gear means rotation of worm causes roation of worm wheel, but the roation of worm wheel can not be transmitted to worm wheel..This is especially useful for lifting applications.

4) Noiseless : The operation of the Worm and Worm gears is smooth and silent.

Following are the drawbacks/Limitations of Worm and Worm gear:

1) Lower Efficiency : The effiiency of the worm gear is low as compared to other types of gears due to more rubbing action.

2) Heat generation : Considerable amount of heat is generated in worm gear drives, which must be dessipated to the surrounding.

3) More friction :   There is more friction between meshing elements,which generates heat as well as there is loss of power, also causes more wear and tear.

4) Costlier gears : Worm wheel is made up of phosphor bronze, which increases the cost.

2) State the applications of worm gears.

Ans :Following are the applications of worm and worm gear drive :

1) Steering mechanism in automobiles

2) Opening and closing of gate valves.

3) Hoists and lifts.

4) In machine tools for driving the tool or work table

5) Speedometer gear to tranmit motion .

6) Converyor belts.

7) Lifts and elevators.

3) Why two dissimilar materials are used for worm and worm wheel? Also state the commonly used material for worm and worm wheeel?

Ans : Practically worm wheel cannot be accurately generated in the hobbing process. The final profile and finish of the worm wheel teeth is the result of plastic deformation during the initial stages of the service. Means the worm wheel must be soft enough to get adjusted as per the worm. Hence it is necessary to make the worm from the harder material and worm wheel from the softer material, so that when they engage the worm makes the required changes in the worm wheel profile.  Generally the worm wheel s madeup of Phosphor-bronze with surface hardness about 100 BHN, whereas the worm (screw) is made up of case hardened steels like 40C4 or 14C4.

4) Explain the following terms for the worm & worm wheels.

i) Diameter quotient

ii) Self locking worm

iii) Rubbing velocity

Ans :

Diameter quotient : Diametral quotient is the ratio of pitch circle diameter of worm to the module of gear tooth .

Self locking worm : Since worm is driver and worm wheel is driven member and the reverse motion from the wheel to worm is not possible hence it is called self locking worm. A worm gear drive is called self-locking if the coefficient of friction is greater than the tangent of lead angle.

where  = coefficient of friction between worm and worm wheel and  = lead angle.

Rubbing velocity :Rubbing velocity is the relative velocity between the worm and worm wheel. the coefficient of friction is the function of the rubbing velocity. when the rubbing velocity is higher there is less coefficient of friction.

5) Explain force analysis for the worm gear drive.

6)  Derive expression for efficiency of worm gear.

7) In design of worm gear, why gear governs the design

8) Write a note on thermal consideration in worm gears

TERMINOLOGY OF WORM GEAR

 A pair of worm gear is specified and designated by four quantities in the following manner

Where z1= Number of starts on the worm, z2= Number of teeth on the worm wheel, q= diametral quotient ( d1/m) , m= module, d1= pitch circle diameter of worm.

Following are the Numerical Problems collected from various University exams

1) A worm -worm wheel are designated as 1/30/10/5, Determine 1) Speed reduction 2) Centre distance 3) Pitch-circle diameters of worm and worm wheel 4)Dimensions of worm 5) Dimensions of gear wheel 

2) A Pair of worm-worm wheel is designated by 1/52/10/8, calculate the following 1) Speed reduction 2) Centre distance 3) Pitch-circle diameters of worm and worm wheel 4)Dimensions of worm 5) Dimensions of gear wheel 

3)  A Pair of worm-worm wheel is designated by 1/30/10/8, calculate the following 1) Speed reduction 2) Centre distance 3) Pitch-circle diameters of worm and worm wheel 4)Dimensions of worm 5) Dimensions of gear wheel 

4) A pair of worm and worm wheel is designated by 2/60/10/5. Worm is running at 1440 rpm and transmitting power of 6 kW. If the coefficient of friction between worm thread and wheel teeth is 0.09, determine the components of gear tooth forces on worm and gear wheel. Take pressure angle as 20 degrees.

5) A pair of worm and worm wheel is designated by 1/31/10/6. Worm is running at 750 rpm and transmitting power of 5000 Watts. If the coefficient of friction between worm thread and wheel teeth is 0.10, determine the components of gear tooth forces on worm and gear wheel. Take pressure angle as 20 degrees.

6) Determine the beam strength and wear strength of a worm wheel drive which is designated by 1/30/10/6. The gear is made up of steel having ultimate tensile strength of 450 MPa and wheel is made up of phosphor bronze having allowable bending stress of 85 MPa. the face width of gear is 0.75 of worm wheel PCD. The worm is running at 1200 rpm and the pressure angle is 20 degrees.

7) A pair of worm and worm wheel is designated as 3/60/10/6. It transmits 5000 watts at 1440 rpm to the worm wheel. If coefficient of friction is 0.1 ,determine the components of gear tooth forces on worm and gear wheel. Take pressure angle as 20 degrees.

8) A pair of worm and worm wheel is designated as 1/30/10/10. The input speed is 1200 rpm. The gear is made up of case hardened steel 14C6 having bending strength 450 MPa and worm wheel is made up of phosphor bronze having allowable bending strength 80 MPa. The face width of the gear is 0.75 of worm wheel PCD. Determine the power transmitting capacity based on beam strength.