Worm Gearbox


Worm gears have a long past and have progressed to advance over theyears. For instance, currently worm gearing is a major factor inhydraulic fracturing, a procedure which has unlocked many natural gasreservoirs. Worm gears use in the process entails conveying power.The gears also apply to power generation industries in coalpulverizers by providing the torque needed in exerting enoughpressure for compressing coal. Other uses include heavy-duty usesthat need speed cutback, in steel mills and large transit structures.The usual worm gearbox comprises of a worm, which meshes with awheel. The paper is a macro process plan for worm gearbox.

Material selection – The materials employed in making gears rangefrom steel. It is also possible to use different non-ferrousmaterials such as composites or plastics. Worm gears are made fromforged steel blanks. The chemical composition is “Fe – basematerial, C – 0.15-0.20 percent, S – &lt0.025 percent, P – &lt0.030percent, Si – 0.17-0.37 percent, Mn – 0.25 – 0.50 percent, Cr -2.8-3.3 percent, Mo – 0.35-0.55 percent, W – 0.30-0.50 percent, Co -0.60-0.85 percent, and Ni – &lt 0.5 percent” (Bawa, 2004). Gearmaterial ought to comprise of certain properties. These are a loftytensile potency, which avoids failure against motionless weights,maximum endurance to handle massive weights, reduced frictioncoefficient and perfect manufacturability. There is no defined timefor selecting materials. The step also includes choosing the pitch aswell as diameter of tap and rod, which will be employed. This followsvalidating the gear diameter blank required to produce the neededteeth number (Sapp, 2014).

Performing blanks – This step involves making a disc. Differentmethods are applicable in making the disc. One is casting, whichcould be sand or die-casting. The gear production technique is thesame as that applied in casting different products. Sand castingapplies when the worm gears are meant for heavy works, however, thegears comprise of poor surface quality and preciseness. Die-castingis applicable when the worm gears are small. The gears are employedin transmitting lightweights (Bawa, 2004). Casting is not expensiveand applies when cost saving is required by manufacturers. Second, isstamping. It suits performing blanks for gears whose sheet metalcomprise of a 3mm thickness. Stamping is achieved via putting thesheet on top of the die used to stamp, which is then pinched througha power press (Bawa, 2004). Third, powder metallurgy refers to aprocedure of gear making via metal powders employing heat and forcewith appropriate binders. It applies to worm gears because the gearsapplied in the automobile industry could be hard to cast. Theprocedure entails the mixing of powdered iron with a specificquantity of graphite powder, which is pushed in dies, as well aspassed via heat to make gears of the needed measurements (Bawa,2004). After which, the gears become drenched with oil, as aprocedure of enhancing the quality of the gears. In general, it ispossible to complete the step within a week, which depends with themethod chosen.

Machining – The step uses blanks normally through roughing orfinishing. The process of cutting gears may be widely categorized toforming and template technique. Forming is founded on the rule ofshaping the cutter tooth to suite the tooth space, which will betaken away (Dudas, 2000). Cutters that have diverse shapes areemployed in cutting every gear size using the provided pitch. It ispossible to apply a single cutter for various gears that have diversefigures of teeth comprising the similar profile, by not interfering alot via operating action. Commercially, every pitch cutter hasdifferent outlines for compensating the changes. During the formprocess, the preciseness of tooth outline relies on the cutter thatis a replica of the tooth space. The precision of worm gears made viaforming relies on the correctness of teeth division, the cutter mustbe properly central to blanks, concentricity of the teeth of the gearto axis, proper tooth space depth, preciseness of the cutter as wellas machine employed, and correct blank indexing. Forming is possiblethrough milling, broaching or shaping.

Milling is a general indexing method applies in cutting gears usingthe milling machine, the machine being vertical or horizontal. Theprocedure entails mounting the cutter to the spindle, which followsrotation over a gear blank placed on a table. The disc or end millkinds of cutters apply in cutting teeth. When employing the disccutter, choosing a cutter relies on the gear tooth measurement andfigure on teeth. End mill cutters relates to cutting gears of hugemodule of more than 20mm, because the cutters use minimal power(Duddas, 2000). When the cutter is done with tooth profile, indexingto the subsequent position follows. The procedure recurs until allteeth have been cut. Milling is advantageous since the startingexpense on cutters is minimal and is applicable in roughing as wellas finishing processes.

Cutting of worm gears on the milling machine happens in two phases.These are roughing and finishing respectively. Gashing happens via aspur gear cutter with the figure of pitch aligning to that of wormgear teeth. The gear blank is placed on an arbor that is held amidthe centres of a dividing head built-in on the bed of a millingmachine. The cutter employed in tied to the milling machine arbor(Gralla, 2007). Proper cutter centering, crosswise as well aslengthwise is important in ensuring gear preciseness. Angle tableswiveling is important to ensure the leaning of gashes aligns withthe worm thread of the helix or lead angle. The finishing operationis executed via a hob following the locating of the machine table incorrect positions to the cutter spindle. The dog and arbor areseparated, followed by unfastening of the headgear to ensureunrestricted spindle rotation (Gralla, 2007). The revolving of thehob happens simultaneously to that of the worm gear. In theprocedure, the gear blank progressively rises to create teeth, whichnet well to that of the hob. Broaching and shaping do not apply tothe process of making worm gears.

Gear generation – the step entails creating tooth flanks, whichacts as the sketch of the ensuing cutter position. In worm gears,gear generation becomes possible via hobbing. Hobbing is the maintechniques applied in producing worm wheel teeth. It involves eitherradial infeeding or tangential feeding, which are both appropriatewhen creating a throated worm wheel (Burdick, 2003). The hobmanufacturer has to be given information concerning the technique ofmaking the worm prior to the designing and creating of the hob. Inaddition is providing the manufacturer with information concerningthe specifications of the worm tooth to ensure the hob creates a wormwheel comprising a tooth profile aligning with the worm. In bothhobbing techniques, it does not involve feeding the hob axially tothe worm wheel and instead, a position becomes fixed. The axial hobplacing during teeth generation determines the placing of the wormaxis. Theoretically, the hob is a duplication of the worm, inreference to “circular pitch, pressure angle and tooth form”(Burdick, 2003).

However, supposing the hob is made to the specification, it becomesunnecessary following its initial re-sharpening, because its sizereduces relative to that of the worm. The outcome of creating a wormwheel via hobbing, which is lesser, compared to the worm is becausein application, the worm may only meet the worm wheel from theexterior teeth edges (Burdick, 2003). Hobs intended for creating wormwheels are made to be bigger compared to the worms by a level, whichpermits several re-sharpenings prior to replacement of the hob. Usinga somewhat bigger hob compared to the worm results in an enhancedcurvature radius, this makes the tooth contact to focus on the wormwheel teeth central point. Another outcome is the necessity of makinga minor rectification on the 90degrees position amid the worm wheelcentral lines and its hob (Burdick, 2003). The hob manufacturer mayprovide data on techniques to apply in measuring the level of angularcorrection needed at any instance all through the hob life.

Inspection – the last step is inspecting the worm gear.Comprehensive inspection, as well as measuring of gears during eachstage and after is important for appropriate and effective gearperformance. Several items need proper checking, which are profile,the thickness of teeth, and spacing between each tooth (Bawa, 2004).The technique largely used in inspection is combination, whichinvolves running the gear in a mesh comprising of the main gearthrough a gear tester. Tools to use during inspection include the“Tooth Vernier Caliper” that determines the gear tooth widthwhile on a pitch circle (Bawa, 2004). The tool comprises beams thathave line scales. Thickness is determined via jaws. Testing happensby calculation of the gear cordal addendum, as well as tooth width onthe pitch circle. The “Base Tangent Method” inspects tooth widthas well. The method’s length equals a single base circular width.It is used to determine if planes are tangent with base circles(Bawa, 2004).

The suitability of a worm gearbox is founded on contact sequence, inaddition to reaction. Inspection of cylindrical worms happens viadirect calculation. In worm wheels, it happens via meshing by amating worm (Burdick, 2003). It can be performed via a general wormgear inspection tool. When preparing for contact inspection, markingcompounds that have different colors are used in the worm and itswheel. This follows meshing of the worm and worm wheel, and rotationto drive the gear. The procedure is repeated up to when worm wheelscreate a single revolution. The ensuing contact sequence is evaluatedto determine the location, width and length (Burdick, 2003). Designspecifications mainly spell out the position of contact sequence,founded on tooth loading, pace, preciseness and use. Contact from thefarthest teeth edges must not happen, specifically when there is nocontact during the central teeth portion. Software is employed ininspecting worms and worm wheels to ensure they meet neededcalculations.


Bawa, H. S. (2004).&nbspManufacturingprocesses. New Delhi:Tata McGraw-Hill.

Bralla, J. (2007). Handbookof Manufacturing Processes.New York: Industrial Press.

Burdick, R. (2003). Manufacturing single-envelope worm gears. GearSolutions. Retrieved from: http://www.gearsolutions.com/article/detail/5748/manufacturing-single-envelope-worm- gears

Dudás,I. (2000).&nbspThetheory and practice of worm gear drives.London: Penton Press.

Sapp, J. (2014). Making worm wheels on the lathe. Retrieved from: http://jimshomeplanet.com/wormgear/wormgear.html