IIJIID~(i1lJ ~~ [~ ~ ~.1. [~ INSTRUCTION MANUAL HTM.25 Gear Trains Apparatus
Gear Trains INTRODUCTION There are two main purposes for using a train of gears. The most important is to establish a speed ratio between two rotating shafts: the other is to transfer rotation from one axis to another with or without a change in the direction of rotation (that is clockwise or anticlockwise). If spur gears are used the axes may be parallel or coincident. The use of bevel gears enables the axes to be at an angle to each other. In the following experiments the speed ratio is the subj~ct being studied. Nevertheles some account will be taken of the direction of rotation as this can be changed independently of the speed ratio. Only simple spur gears will be used, although an introduction to the principles of epicyclic gear trains is included in an elementary form. Hence there are several aspects of the application of gears and gear trains to be learned by doing this experiment. LIST OF PARTS The standard set of items supplied (HTM.2S) consists of: - Gear trains apparatus clw four spur gears and axle pins - Box of washers and nuts APPARATUS A pivoted slotted arm mounted on a horizontal base carries two movable short axles on which four interchangeable spur gears can be arranged. One pair of the gears can be pinned together in order to assemble a compound gear train. The slotted arm can be locked in position for gear train HTM25. Page I. Issue I..January, 1994.
~ work, or it can be rotated about its pivot to simulate the planet ring of an epicyclic system spur gears have 40,60,80 and 100 teeth with 32 diametral pitch and a 200 pressure angle. EXPERIMENT There are two separate experiments to be performed on this apparatus, one on gear trains and the other an introduction to epicyclic gears. OBJECT No.1 The object is to study the speed ratios and directions of rotation of simple and compound gear trains PROCEDURE The Part Simple gear trains. Spacer, MeciumPWI Any two Gear Wheels " Short Pm vwi1gnut Assemble a two wheel gear train with the 80 tooth wheel as the driver on the fixed pivot as shown in diagram A. Mark the teeth of both gears at the point where they mesh. Turn the driver through one revolution clockwise and note how many revolutions (and parts of a revolution) the driven wheel executes. As an alternative keep turning the driver through enough complete revolutions for the two marks to corne together again. and note the corresponding number of complete turns of the driven wheel Record the direction in which the driven follower wheel rotates. Repeathe above procedure using the other two gear wheels in turn Medium Pin "" " 1/ Spacer A - Medium Pin / Shcxt p., / Any three gears / / Washer ""J- Wingnut Diagram 8
IIT,\f25. Page 3. \"Ue I. May, 1994. Next assemble a three wheel gear train as in diagram B with the 80 tooth driver and the 100 tooth wheel at the ends of the train. Note the initial positions of marks on the driver and final wheel of the train, and then determine the turns ratio by one of the above methods. Also note the direction of roation of the final wheel when the driver turns clockwise. Repeat this work using the other gear wheel as the idler between the driver and follower. Part 2. Compound gear trains. Keeping the 80 and 100 tooth wheels as driver and final wheel respectively, arrange a compound gear train as shown in diagram C. Note the positions of the marks on these two wheels and then proceed to determine the turns ratio and corresponding directions of rotation. Repeat this with the intermediate pinned pair the other way up. RESUL TS Compare the experimental results with the theoretical predictions Nr; No : To TF where N= speed (revolutions/unitime) r = teeth on gear wheel D stands for driver F stands for follower For the three wheel train the fqrmula becomes NI. T",, NF% and N1 = ~~ TF T1 ND. To NF= TF The compound gear train is resolved in a similar manner. designated A and B, and the second set C and D Then Let the first driver and follower be
~ ButN. N r. hence ND: NA1A T(..: -r;;-e-r:: OBSERVATIONS What are the differences between a simple two and three wheel gear train? Predict what would happen if a four wheel train using two idlers was set up, What advantage is there in using a compound gear train? OBJECT No.2 The purpose of this experiment is to study the first principles of an epicyclic gear system with particular reference to the speed ratio and direction of rotation PROCEDURE [n its simplest or typical fonn an epicyclic system comprises a central (sun) wheel around which a planet wheel or, for better balance, wheels circulate while an outer ring gear with the teeth pointing inward meshes with the planet wheels as in the figure shown below..... r Outer ring v, ~ ", Planet Wheel Planet wheel frame ~=~/, Sun wheel :, Experimental simulation k-- of outer ring To simplify the design for the purpose of the experiment the planet wheel frame is a single arm that revolves on the same axis as the sun wheel (but independently) while the outer ring is represented by a normal spur gear (with many less teeth).
Part 1. Planet Wheel. L... Short pin Washer -~~- - Wingnut Diagram D Set up the 80 tooth wheel as the sun wheel with the 40 tooth wheel in mesh as a planet wheel The ann carrying the planet wheel should be in its "locked" position. See diagram D. As the velocity ratio of an epicyclic gear train is not easily observed when one considers the simultaneous movement of gear wheels and arm, it is usually better to study the relative movement of the gear wheels with the arm stationary, then the revolution of the gear trains without any relative motion of the wheels and, finally, the combination of these two operations. This system of establishing the relative motion of the wheels and arm is illustrated in the following arrangements where the sun wheel is designated A and the planet B. Use table I for recording the revolutions and direction of rotation taking clockwise as +ve. Table 1 Relative motions of 80 tooth sun & 40 tooth planet "'heels Mark the sun and planet wheels at their meshing point. With the arm fixed turn the sun one revolution anticlockwise and observe the number of revolutions of the planet wheel and their direction. Next clamp the sun wheel to the arm and rotate the arm one turn clockwise. Watch the marks on the sun and planet wheels to determine their rotation with respect to the axes of each wheel (it may help to follow the mark on the planet wheel with one's finger). Finally rotate the arm while holding the sun wheel stationary and count the number of revolutions of the planet wheel and note their direction Repeathe whole procedure using the 100 tooth wheel as the sun wheel ~ H1:\f.:?5. Page 5. f.\:\"ue I I\.JO.V. /994.
H1M. loup I Part 2. Simple epicyclic gear. Diagram E Add an 80 tooth "outer" ring wheel C to the 100 sun and 40 tooth planet wheels as shown in diagram E. The experiment is to detennine the speed and direction of rotation of wheel C when A turns one revolution clockwise while the planet arm makes one anticlockwise turn. Follow through a similar procedure to the above, starting with the arm stationary in table 2. Table 2 Relative motions of 100 tooth sun, 40 tooth planet & 80 tooth outer The final action is to simultaneously rotate the sun wheel once clockwise while turning the ann once anticlockwise, and to count the number of revolutions of the outer wheel. Also note the direction in which C turns. Part 3. Compound epicyclic gear. Diagram F
In'M.25. IJage 7. s-ue /.,\.fav. /994. Assemble a gear train using the 100 tooth wheel as the sun meshing with the 40 tooth gear of a pinned 40/60 pair in the planet position. Then use the 80 tooth wheel meshing with the 60 gear to provide the "outer ring", as in diagram F. Repeathe experiment described in Part 2. Finally determine the speed and direction of the sun wheel when the arm makes one anticlockwise turn while the 'outer ring' is stationary (relative to the base of the apparatus - NOT the arm). A table like 2 above can be used. RESULTS Produce formulae for the spaces in the tables and compare the experimental results with the theoretical. OBSERVATIONS In the case of a real epicyclic gear system the outer ring must inevitably have a large number of teeth. Estimate how many teeth it would have been for the simple epicyclic gear of Part 2, and hence determine the speed ratio.