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Articles About worm gear drives
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How does one determine the center of a worm and a worm wheel? Also, what are the differences between the common worm tooth forms?
Transmission of power between nonparallel shafts is inherently more difficult than transmission between parallel shafts, but is justified when it saves space and results in more compact, more balanced designs. Where axial space is limited compared to radial space, angular drives are preferred despite their higher initial cost. For this reason, angular gear motors and worm gear drives are used extensively in preference to parallel shaft drives, particularly where couplings, brakes, and adjustable mountings add to the axial space problem of parallel shaft speed reducers.
Friction weighs heavily on loads that the supporting journals of gear trains must withstand. Not only does mesh friction, especially in worm gear drives, affect journal loading, but also the friction within the journal reflects back on the loads required of the mesh itself.
We need a method to analyze cumulative fatigue damage to specify and to design gear drives which will operate under varying load. Since load is seldom constant, most applications need this analysis.
Recently, there has been increased interest in the dynamic effects in gear systems. This interest is stimulated by demands for stronger, higher speed, improved performance, and longer-lived systems. This in turn had stimulated numerous research efforts directed toward understanding gear dynamic phenomena. However, many aspects of gear dynamics are still not satisfactorily understood.
Chairman Todd Praneis of Cotta Transmission describes the activities of AGMA's Enclosed Drives technical committee.
An experimental and theoretical analysis of worm gear sets with contact patterns of differing sizes, position and flank type for new approaches to calculation of pitting resistance.
Worm gears display unique behavior of surfaces because of the presence of wear phenomena in addition to contact pressure phenomena.
Solutions to the governing equations of a spur gear transmission model, developed in a previous article are presented. Factors affecting the dynamic load are identified. It is found that the dynamic load increases with operating speed up to a system natural frequency. At operating speeds beyond the natural frequency the dynamic load decreases dramatically. Also, it is found that the transmitted load and shaft inertia have little effect upon the total dynamic load. Damping and friction decrease the dynamic load. Finally, tooth stiffness has a significant effect upon dynamic loadings the higher the stiffness, the lower the dynamic loading. Also, the higher the stiffness, the higher the rotating speed required for peak dynamic response.
There's a reason they call it catastrophic gear failure: For example, if the line goes down at a large aluminum rolling mill because a gear set goes bad, the cost can run up to a whopping $200,000 a week. Even in smaller operations, the numbers alone (not to mention all the other problems) can be a plant manager's worst nightmare.
Wave generators are located inside of flexsplines in most harmonic gear drive devices. Because the teeth on the wheel rim of the flexspline are distributed radially, there is a bigger stress concentration on the tooth root of the flexspline meshing with a circular spline, where a fatigue fracture is more likely to occur under the alternating force exerted by the wave generator. The authors' solution to this problem is to place the wave generator outside of the flexspline, which is a scheme named harmonic gear drive (HGD) with external wave generator (EWG).
It has been documented that epicyclic gear stages provide high load capacity and compactness to gear drives. This paper will focus on analysis and design of epicyclic gear arrangements that provide extremely high gear ratios. Indeed, a special, two-stage planetary arrangement may utilize a gear ratio of over one hundred thousand to one. This paper presents an analysis of such uncommon gear drive arrangements and defines their major parameters, limitations, and gear ratio maximization approaches. It also demonstrates numerical examples, existing designs, and potential applications.
Gearbox performance, reliability, total cost of ownership (energy cost), overall impact on the environment, and anticipation of additional future regulations are top-of-mind issues in the industry. Optimization of the bearing set can significantly improve gearbox performance.
The primary objective in designing reliable gear drives is to avoid failure. Avoiding failure is just as important for the manufacturer and designer as it is for the end user. Many aspects should be considered in order to maximize the potential reliability and performance of installed gearing.
Several articles have appeared in this publication in recent years dealing with the principles and ways in which the inspection of gears can be carried out, but these have dealt chiefly with spur, helical and bevel gearing, whereas worm gearing, while sharing certain common features, also requires an emphasis in certain areas that cause it to stand apart. For example, while worm gears transmit motion between nonparallel shafts, as do bevel and hypoid gears, they usually incorporate much higher ratios and are used in applications for which bevel would not be considered, including drives for rotary and indexing tables in machine tools, where close tolerance of positioning and backlash elimination are critical, and in situations where accuracy of pitch and profile are necessary for uniform transmission at speed, such as elevators, turbine governor drives and speed increasers, where worm gears can operate at up to 24,000 rpm.
The type of lubricant and the method of applying it to the tooth flanks of large open gears is very important from the point of view of lubrication technology and maintenance. When selecting the type of lubricant and the application method, it is important to check whether it is possible to feed the required lubricant quantity to the load-carrying tooth flanks, This is necessary to avoid deficient lubrication, damage to the gear and operational malfunctions. It is important to determine the type of lubricant, which may be fluid or grease-like. The consistency of the lubricant will have a direct impact on the ability of the lubrication system to feed adequately the lubricant to the gear. The interactions between the common types of lubricant and the lubrication application methods for open gear drives are shown in Fig. 1.
This method of testing large gearboxes or, indeed, any power transmission element, had numerous advantages and offers the possibility of large savings in time, energy, and plant, if the overall situation is conducive to its use. This usually requires that several such units need to be tested, and that they can be conveniently connected to each to each other in such a way as to form a closed-loop drive train. No power sink is required, and the drive input system has only to make up power losses. The level of circulating power is controlled by the torque, which is applied statically during rotation, and the drive speed. Principles, advantage, and limitations are described, together with recent experiences in the only known large-scale usage of this technique in Australia.
Worm gears are among the oldest types of gearing, but that does not mean they are obsolete, antiquated technology. The main reasons for the bad experiences some engineers have with worm gearing are misapplication and misuse. No form of gearing works for every application. Strengths and weaknesses versus the application must be weighed to decide which form of gearing to use. For proper application and operation of worm gears, certain areas that may differ from other types of gearing need to be addressed.
Rules and Formula for worm gears, bevel gears and strength of gear teeth.
Among the various types of gearing systems available to the gear application engineer is the versatile and unique worm and worm gear set. In the simpler form of a cylindrical worm meshing at 90 degree axis angle with an enveloping worm gear, it is widely used and has become a traditional form of gearing. (See Fig. 1) This is evidenced by the large number of gear shops specializing in or supplying such gear sets in unassembled form or as complete gear boxes. Special designs as well as standardized ratio sets covering wide ratio ranges and center distanced are available with many as stock catalog products.
Worm gearing is of great antiquity, going back about 2100 years to Archimedes, who is generally acknowledged as its inventor. Archimedes' concept used an Archimedial spiral to rotate a toothed wheel. Development of the worm gearing principle progressed along conventional lines until about 500 years ago when Leonardo DaVinci evolved the double enveloping gear concept.
A very direct and effective way of increasing power transmission efficiency is a changeover from mineral-oil-based lubricants to synthetic lubricants.
There is an increasing significance of screw helical and worm gears that combine use of steel and plastics. This is shown by diverse and continuously rising use in the automotive and household appliance industries. The increasing requirements for such gears can be explained by the advantageous qualities of such a material combination in comparison with that of the traditional steel/bronze pairing.
The effect of various lubricant factors on wormgear efficiency has been evaluated using a variety of gear types and conditions. In particular, the significant efficiency improvements afforded by certain types of synthetic lubricants have been investigated to determine the cause of these improvements. This paper describes broad wormgear testing, both in the laboratory and in service, and describes the extent to which efficiency can be affected by changes in the lubricant; the effects of viscosity, viscosity index improvers and, finally, synthetic lubricants are discussed. The work concludes that lubricant tractional properties can play a significant role in determining gear efficiency characteristics.
Worm gear speed reducers give the design engineer considerable options, but these gear systems present a challenge to the lubrication engineer. Heat energy generated by the high rate of sliding and friction in the contact zone causes worm gears to be relatively inefficient compared to other gear types. Because worm gears operate under a boundary or near-boundary lubrication regime, a satisfactory lubricant should contain a friction modifier to alleviate these conditions.
Eliot K. Buckingham explains the procedure for proper measurement over wires for worm gears, in response to last issue's article.
Question: When cutting worm gears with multiple lead stock hobs we find the surface is "ridged". What can be done to eliminate this appearance or is to unavoidable?
Question: Do machines exist that are capable of cutting bevel gear teeth on a gear of the following specifications: 14 teeth, 1" circular pitch, 14.5 degrees pressure angle, 4 degrees pitch cone angle, 27.5" cone distance, and an 2.5" face width?
Question: When evaluating charts from a gear inspection machine, it is sometimes found that the full length of the profile traces vary, and that sometimes they are less than the length of active profile (above start of active profile-SAP) by up to 20%. This condition could be caused by a concentricity error between tooth grinding and shaping, or by unequal stock removal when grinding. (See Fig. 1.) Is it possible that some of the variation is coming from the inspection machine? How can variation from the inspection machine be reduced?
Two questions on hobbing cover the various types of hobs and their unusual names, as well as the importance of hob swivel angle.
The dimensions of the worm and worm gear tooth surfaces and some of the worm gear drive parameters must be limited in order to avoid gear undercutting and the appearance of the envelope of lines of contact on the worm surface. The author proposes a method for the solution of this problem. The relations between the developed concept and Wildhaber's concept of the limit contact normal are investigated. The results of computations are illustrated with computer graphics.
We make a lot of single-start worm and worm gear sets, and it always seems as though we're buying another special hob. We also do a lot of spur gear cutting, and the spur gear hobs and the worm gear hobs look alike, so we wonder why we cannot use the standard hobs for cutting worm gears too. Can we do this?