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Production technology is currently undergoing a transformation, known in industry and science as Industry 4.0

Image: Klingelnberg

Gear Cutting Lapping or grinding: The right choice?

Dec 18, 2017

Bevel gear transmissions for the automotive industry are subject to extremely stringent requirements. They must be able to transmit ever greater outputs at lower weights, in less space. Noise quality and efficiency must measure up to high standards. During bevel gear teeth dimensioning, a decision must be made as to whether the gearing will be lapped or ground. The article highlights the advantages and disadvantages of both these methods.

Production technology is currently undergoing a transformation, known in industry and science as Industry 4.0. Production processes must meet the ever-growing requirements for flexibility and self-optimisation. This calls for information to be gleaned from the processes and be directly manipulated by the production systems. Against the background of this discussion on the benefits of Industry 4.0, it is worthwhile to re-examine an old question in bevel gear production: Should bevel gears be lapped or ground?

At first glance, the process chains for ground and lapped bevel gear teeth differ only in hard finishing. The geometry changes that can be achieved with generation-lapping are significantly smaller than with gear grinding. Consequently, the final quality of a lapped bevel gear depends to a far greater degree on the result of the preceding process steps. For production, this means that significantly more effort is required to optimise the component quality during gear-cutting. This includes an allowance for geometry changes brought about by heat treatment, usually case hardening. The so-called hardening distortion compensation is required because these distortions can only be corrected to a limited extent by lapping. As regards to ground gearing, by contrast, due to a significantly greater material removal compared with lapping, much greater amounts of hardening distortions can be eliminated.

Features of lapped bevel gear teeth

Due to the shorter gearing times, lapped gearings in mass production are mostly manufactured in a continuous process (face hobbing). These gearings are characterised by a constant tooth depth from the toe to the heel and an epicycloid-shaped lengthwise tooth curve. This results in a decreasing spacewidth from the heel to the toe. During bevel gear lapping, the pinion undergoes a greater geometric change than the gear since the pinion experiences more meshing per tooth due to the smaller number of teeth. Material removal during lapping results in a reduction of lengthwise and profile crowning, primarily on the pinion, and to an associated reduction of the rotational error.

As a result, lapped gearings have a smoother tooth mesh. The frequency spectrum of the single flank test is characterised by comparatively low amplitudes in the harmonic of the tooth mesh frequency accompanied by relatively high amplitudes in the sidebands (noise). Indexing errors in lapping are reduced only slightly and the roughness of the tooth flanks is greater than that of ground gearings. One characteristic of lapped gearings is that each tooth has a different geometry owing to the individual hardening distortions of each tooth.

Features of ground bevel gear teeth

In the automotive industry, ground bevel gears are designed as duplex gearings. A constant space-width and an increasing tooth depth from the toe to the heel are geometric features of this gearing. The tooth root radius is constant from the toe to the heel and can be maximised due to the constant bottom land width. Combined with the duplex taper, this results in a comparable higher tooth root strength capability.

The uniquely identifiable harmonics in the tooth mesh frequency, accompanied by barely visible sidebands, are significant attributes. For gear cutting in the single indexing method (face milling), twin blades are available. The resulting high number of active cutting edges increases the productivity of the method to an extremely high level, comparable to that of continuously cut bevel gears. Geometrically, bevel gear grinding is an exactly described process, which allows the design engineer to precisely define the final geometry. To design the EaseOff, geometric and kinematic degrees of freedom are available to optimise the running behaviour and load capacity of the gearing. Data generated in this way is the basis for the use of the quality closed loop, which in turn, is the prerequisite for producing the precise nominal geometry.

The geometric precision of ground gearings leads to a small variance between the tooth geometry of individual tooth flanks. The indexing quality of the gearing can be significantly improved by bevel gear grinding.

Influence of hard finishing on gear set development

The geometry of lapped gearings is the result of an iterative development. The design engineer ultimately specifies the final geometry of the gear set on a conditional basis since the final quality variance in lapped bevel gear teeth is greater than for ground gearings. As a result, the influence of production on running performance is also greater than is the case for ground bevel gear teeth. This leads to a greater uncertainty in the gear set development since the design engineer must continually evaluate the production influence and quality of the design.

The high geometric accuracy of ground bevel gear teeth allows for clear feedback during the gear set development of gearing performance with respect to geometric design. The design engineer receives clear feedback as to which geometric variations result in poor running behaviour or insufficient load capacity of the gearing and gleans information from this for optimised gear geometry.

Quality closed loop

An important part of the process chain for bevel gear grinding is the quality closed loop. During gearing design, the design engineer uniquely defines the nominal geometry of the ground gearing. A virtual master and virtual machine model form the basis for the Klingelnberg quality closed loop. On the precision measuring center, deviations in the ground gearing are measured against a virtual master. Based on the model of the virtual cutting machine, correction data is calculated from the real deviations and the grinding process is tuned. Thus, the closed loop describes a self-optimising system and is a good example of Industry 4.0.

In the process chain for lapped gearings, the closed loop is also used for quality optimisation of the soft cutting process. Compared with the ground gearing process chain, however, a virtual description of the lapping process does not exist. For this reason, self-optimisation of the bevel gear lapping process with a virtual master is not possible. With lapping, the operator is still an elementary component of the quality closed loop.

In the process chain for ground bevel gear teeth, all Klingelnberg cutting machines are networked with the production database. The closed loop for bevel gear cutting comprises of three process steps: blade grinding, cutter head setup and bevel gear cutting. The bevel gear cutting process can be supplemented with an optimisation loop to allow for hardening distortions. In this case, the virtual master for the quality closed loop for gear cutting is corrected. The hardening distortion correction can be eliminated for ground gearings since the process is insensitive to the input quality. Moreover, bevel gear grinding is optimised via a dedicated quality closed loop. If fluctuating hardening distortions in production influence the quality of the ground component, this influence is eliminated in the closed loop for bevel gear grinding. This is not possible for lapping.

A question of application

The choice of hard finishing is primarily a question of the application, even in the age of Industry 4.0. Owing to the tooth form and the quality that can be achieved, bevel gear grinding is well-suited for transmissions subjected to very high loads and for extreme noise requirements. Likewise, in the case of strong variance in hardening distortion and flexibility requirements in production, bevel gear grinding is preferable to bevel gear lapping, to avoid disproportionately large expenditures for hardening distortion compensation.

Another key advantage is production in a self-optimised closed loop which enables setup of decentralised production networks to ensure the same high production quality across all sites. In the future, networking of decentralized production facilities in a global production network will be the success factor for producing top-notch quality in a cost-effective manner, regardless of location.

The article is reproduced with due courtesy to Klingelnberg

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