A Clockwork Gear

IQuestion:: Could you explain whatiismeant by "borologicaEgealiing"? Inever heard of this befor,e,altboughI understand. it has something to dowith watches. Could you also explainthe mea.ning of a.!!going gear train"?

Horology i the science of measur-ing time or the art of constructing in-struments that indicate time. In ear-lier days this was as simple as track-ing the sun using a sundial; todaywe use the vibration of a quartzcrys-tal. For ultra-precise timing. theatomic clock uses a resonant fre-quency of Cesium 133 and i accu-rate to one econd in 250',000 years.Not quite what you need around thehouse for tellmg time!

Although sundials can get quitesephisticated, they utilize no gears;neither do most modern. digital tim-ers and watches. But for the severalcenturies in between, docks, watches,timers, fuses. and other such deviceswere prolific users of gears. An entirefield of design and manufacture oftile e mechanical gear element. de-veloped, and all the gears used inclock: , watches, and other timing de-vices, whether for driving the point-ers, setting the hands, winding upsprings, or driving the escapementmechanism and pendulum, were called"horolo gicalgearing' .

Many books were written that cov-ered the special concerns involved inthis application of gearing, and theserious student could even enroll incorrespondence courses in "horologi-cal. science."

The problem of design faced bywatch builders, especially consider-ing the very small size of some me-

William l,JanOlinc:k

chanical watches, were totally dif-ferent from those experienced bypower train designers in transporta-tion. construction, machine tools.and general machinery applications ..

Some of the problems facing thehorological gear designer come fromthe necessity of using multiple-stage,high-ratio. step-up gear sets. They axethe opposit,e of the more usual appl ica-tion problems of step-down or speedreducing drives faced by most de ign-ers ..In addition is the ab clute meed forlow friction and high efficiency.

In a mechanical watch or clock, tiledriving 'energy is stored in a spring-wound drum and is slowly released asminute pulses through the tep-updrive train and into an escapementmechanism, with the gear train. tatt-ing and stopping completely with eachcycle of the escapement wheel. Thefrictien through the gear train must beminimal for tile uccessful managementand release of the available energy,

One of the first areas of concern instep-up drive is gear tooth friction.To reduce surface contact friction. sur-face finishes must be extremely good,and frequently the pinion teeth are pol-ished. The reduced friction experiencedin tile arc of rece sion, or exit path outof mesh. rather than the arc of ap-proach. or entry path into mesh, is LIsed.To accomplish this, a cycloidal toothform is used, and to further reduce theapproach actiencthe cycloidal toothform on the pinion addendum is modi-fied. concentrating tile action near thepitch line of the gear set

For compactness. pinions of as fewas 6,.7, or 8 teeth and tep-up ratios ofas much as 12: 1 in a single mesh are

Address your gearing questionto, our panel of exparts. Writeto them care' of ShOiP' IF,I"o'or.Giear T,echnology. P.O. Box14.26, IEllk IGmve, Vmag,e, IIIL60009 .•Dr call our editorial staffat 1(7:08) 437-'6604.

used. The teeth can be as fine as 250diamerral pitch wlth depths aslittle as..0 [2". To avoid friction losses fromside thrust, the teeth are alway spur.

Many (oath forms and proportionshave been established suitable forhorological purposes. Some of theseu ed in tep-up drives are the Ogival- orm. British Standard 978. Black For-est Clock Standard, the Swiss Cycloi-dal, the Prescot. and the Circular Arc.Another one frequently encountered

William L Janninckdoes gear and tool designand consulting. He has been.involved will, gears and gearmanufacturingfor 45 years,40 of them with Illinois Tools·/TlV Inc. He is till' authoro]numerous articles .0" gear-related topics.

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Reader Service PageNumber Number

28 47I Cover 2

21 4617 44

9.1.0 6,730 4640 3915 1.54 412 1222 46]9' .518 238 428 10

39 395, 7 Cover 4. II11 813 1416 4325 4614 402 124 4626 466 .520 Cover 341 15

42 G EAR TEe H N 0 LOG V

is the Wickenburg form.Fig, I shows a typical cycloidal form

pinion and gear .ill mesh. The flanks onboth members are radial lines belowthepitch line. A special type of pinionhaving six teeth or more and u ed instep-up drives is called a lantern pin-ion and is shown in Fig. 2. A toothlayout is shown ill Fig. 3. The teeth ateformed of polished pins or wires set ina pair of end plate . These pinions arenot recommended for reduction drivesin horologicaJ applications,

The number 'Of the tip modificationused on clock gears may cause one toquestion the functionality of thesegears, since the tooth form departsfrom true conjugacy. CycIoidal gearing is that 'One exception to the rulerequiring a contact ratio of at least 1.0for a pair of gears to be used success-fully, Contact ratios of less than I .0 oninvolute gearing are a ignal of prob-lems and, with an involute profile, indi-cate a pos ible damaging edge contactand lack of proper uniform transrnis-

Fig. I - Cycloidal gear set.

Fig. 2 - Lantern pinion.

• ••~~r.r!__/J- U P L-

+

Fig. 3 - Lantern pinion gear set.

sian of rotation. Thi i not the casewith cycloidal profiles. and edge COlll-

tact is rare ..A step-lip drive in a. dock is one

application where efficiency of en-ergy tran mi ion, rather than uni-form transmission of rotation. is theprimary objective. If these gears werein peered by the single flank method,the results would how a tooth-to-tooth rotational 'error, but this is not aconcem for clock and watch gearing.

Not all the gears ued in norolog-ical gearing have the cycloidal toothform. Many applications can and douse involute gearing. such a those forwinding drives. time setting, minute-

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SHOPFLOORhom hand synchronizing, or olher mo-

lion transfer. In electric clocks using a.synchronous electric motor powersource, the gearing j . all in reductionstages, and most use invohite gearing.Electric meters used in measuring en-ergy may u e some special high-re-duction worm drives and thenend upwith involute gearing for the dial re-cording drives, Liquid and ga metersalso lend to use involute gearing.

It might be helpful 10 diagram thegear trains u ed in .a typical, hand-wound chiming mantel clock. whereseveral clock gearing applications canbe shown and explained. Fig. 4 showsthe main clock. drive u ed for preci etime keeping and display . Starting withthe pring drum gear there are fivestep-up cycloidal tooth form stageslocated 011 sixaxes. The last stage(Continued on page 48)

MINUTE HAND

Fig. 4 - Main clock train.

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JANUARY/FEBRUARY IU243

pinion ending on the sixth axis drivesthe time controlling mechanism, con-sisting of the escapement wheel and arotary pendulum, while axis three isthe take-off to directly turn the minutehand. This also drives another twostages in reduction to synchronize and

drive the hour hand ..Fig. 5 diagrams the second segment

of the clock, the quarter-hour chimingsection. It also has five stages of step-up cycloidal gears running from thespring drum to an air paddle type speedgovernor. From axis number three,motion is taken off and passed througha chain of four idler gears to drive aset of chime cams. These idler gearsare conventional and utilize typicalinvolute gears.

SHOPFLOOR6 4

Fig ..5 - Quarter-hour chime train.

6SPRING DRUM

AIRPADDLE

HOUR CAM

Fig. 6 - Hour-striking train.

In Fig. 6 the third section of theclock, used for striking the hours, isshown. It too uses a five-stage, step-upcycloidal train, starting with the drumand ending with another air paddlespeed governor. There is a take-offfrom axis number three, and it turns thehour striking cam.

:48: G EAR TEe H N 0 LOG Y

All in all, excluding cams, ratchets,and escapement parts, there are 38gears in this clock.

Another name for the step-up drivedescribed above is a going gear train.As shown in our dissection of the man-tel clock, the going trains are five stagesof step-up gearing using driven pinionsof very small numbers of teeth.

In the above case, the energy wasstored in hand-wound springs, but thiscan be accomplished by other means. Asmall, geared electric motor can wind asmaller, lighter spring, or an electricalsolenoid and a ratchet can do the samething. Another interesting design usesa sealed air chamber, which expandsand collapses with atmospheric pres-sure fluctuations to do the winding.One of the oldest methods uses gravity

ements of profile, tooth thickness,and adjacent tooth spacing, of thesevery fine-pitched, cycIoidal, profiledgears and pinions, can best be doneby optical means, using magnifiedprojected images and by comparingthe shadow against enlarged toothlayouts. Any visible bumps or hol-lows are cause for concern,especiallyin the immediate area of the pitchline. Ifthe inspection layout includesseveral teeth, then any adjacent spac-ing errors can be judged. Surface fin-ish can also be assessed by usingmicroscopes and comparison samples.

In the clock gear field, backlashalso requires special consideration. Thedust and fiber particles that can getinto gear teeth are larger in compari-son to the small teeth in watch applica-

by means of a weight, a rope, and a tions, Provisions must be made forpulley to store the energy. space for this debris to settle without

binding. Oils and greases catch andhold the debris and are not normallyused on the very fine pitches.

The suggestion has been made that.all of this special technology and pro-cedure are not really necessary and anydifference between clock: gearing,andthe more common involute gearingmight be small. I personally becameaware of the significant difference be-tween them many years ago. We had adamaged drive gear set in a timingdevice which we replaced on an emer-gency basis, cutting the gears OUf-

ing force is quite high, and a pair of selves, using an available fine-pitch,step-up stages will suffice, Those gages involute hob ..After all, we thought, aIhave seen utilize involute gearing. gear is a gear as long as the tooth ratio

Mechanical dial indicators usually is correct. We could not make thisapparatus work no matter how muchbuffing, polishing, oiling, and prod-

Except for the very small teeth and ding we did. Finally, in utter frustra-tion, as a last, desperate resort, theoriginal tooth forms were copied, atsome expense, and upon assembly, themechanism took offrunning. We werepretty well convinced. 1.1

To address questions to Mr. William L.Janninck; please circle Reader ServiceNo. 78.

Another interesting application isseen in the traditional mechanical aner-oid altimeter that. is required on allaircraft, at least. as a back-up instru-ment. It uses cycloidal gearing in astep-up train. The gears are of higherquality or precision and usually usehighly polished pinions. The smallmovements of the aneroid chamber aremagnified through the gearing, driv-ing the indicating pointers.

Pressure measuring gages driven bya Bourdon tube sense the flexure of tbetube to indicate the pressure. The driv-

use geared step-up drives, and thesetoo are of involute form.

small size of the gears, the usual manu-facturing methods are used ..Small bob-bing machines using small cycloidalhobs and automated form milling ma-chines using miniature rotary formmilling cutters are employed. Most ofthese machines are of Swiss, German,or English origin,

Inspection, particularly for the el-