Reducing Gear Noise While Improving Performance

Gear noise reduction can be achieved through electrochemical machining, which is a process that accomplishes metal removal by dissolution of surface atoms without direct contact between the tool and workpiece material.

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To protect human health and the environment, noise emission from road vehicles is limited by legislation. In Europe, essential noise limits continue to be defined by Directive 70/157/EEC and its numerous amendments.

According to the Brussels-based advocacy organization Transport & Environment, traffic noise is one of the most widespread environmental problems in the European Union. Noise interferes with people’s daily activities at school, at work, and at home. It causes sleep disturbance, hearing damage, and even cardiovascular disease, and it can hinder worker performance and children’s learning.

Studies have revealed that 50,000 deaths and approaching a quarter of a million cases of cardiovascular disease every year in Europe are linked to traffic noise. For the first time, noise has also been linked to an increased stroke risk. In Denmark, 5 percent of all stroke cases are caused by traffic noise.

In terms of the burden on health, environmental noise is second only to air pollution according to the World Health Organization. Noise standards for road vehicles have not been updated for 20 years and have had little impact on noise levels adjacent to roads. Improved standards are long overdue.

Cutting road noise levels by just 3 decibels is equivalent to halving the level of traffic. This change would deliver a major improvement in the health and quality of life of the vast majority of European citizens who live in towns and cities or near major roads. A 3-decibel reduction is quickly and easily achievable through technologies already readily available to car and truck manufacturers. Nearly one in four of the cars and one in three of the light trucks tested over the past five years already meet the strictest standards proposed by the European Commission. Figure 1

Failing to tackle noise from vehicles at the source requires national governments, local authorities, and homeowners to install noise barriers or sound insulation to homes and public buildings. Quieter vehicles reduce the need for expensive noise abatement and would increase property values since homes in noisy areas are less attractive to potential buyers.

Comparing traffic volume from 1998 and 2010 revealed an increase of up to 40 percent with a corresponding increase of road traffic noise emissions by 1.5 dB(A). This means that efforts for future reduction of road traffic noise must also compensate for steadily growing traffic volume.

There is a continuing desire for traffic noise reduction at a similar rate of approximately 3 dB(A) per 10 years. Most sources agree, however, that achieving this via further per-vehicle legislation will be difficult if not impossible. An integrated approach is called for involving per-vehicle reductions, optimizing the road surface and tire interaction, improved traffic management, and local noise control measures such as noise-absorbing surfaces. %%0815-KM-2%%

Vehicle Noise Generation
Sources of vehicle noise differ depending, in part, on vehicle speed. While powertrain noise dominates at lower speeds, noise generated by tires and road interaction dominates at higher speeds. Improving noise emissions at lower speeds, particularly in first and second gear, means improving powertrain components and minimizing gearbox vibrations.

One method of improvement, particularly at lower speeds, involves putting a radius on the gearing end side. This minimizes risk of any remaining particles (burrs, secondary burrs, or particles) causing unwanted vibrations. Not only are noises and vibrations reduced, but such enhanced gears will also operate at higher efficiency and have longer service life.

Improving Gear Life by Removing Stress Risers
Manufacturers of gears are also looking to enhance the reliability of their components. It is extremely painful and costly to replace a broken gear. Imagine replacing an automatic gearbox fitted in a high-end car or any number of gears involved in wind-energy turbines. The cost impact is much higher than the faulty gear cost. Complete system, manpower, freight, and more make significant costs a sure thing, not to mention the potential damages to a company’s reputation or safety performance record.

Stress Finite Element Method (FEM) analysis of gear teeth have shown that stress load is concentrated at the edge extremities and that non-uniform stress load will, over time, damage the teeth and the complete gear assembly as the material breaking from the surface will destroy other areas. As damages progress, the assembly will generate more and more vibration and noise, contaminating the surrounding components, lubricant, and more.

Electrochemical Machining (ECM)
ECM is a process that accomplishes metal removal by dissolution of surface atoms without direct contact between the tool and workpiece material. This method of selective metal removal follows Faraday’s law of electrolysis, that is, the amount of material removed is proportional to the time and intensity of an electrical current flowing between tool and workpiece. The process is highly controllable and can machine or polish areas previously unreachable by other methods, including hand polishing and deburring.

As an electrolytic solution (water and specialized salts) is pumped over the workpiece surface and a discontinued current (DC) flows between the tool and the workpiece. The amount of material removed is determined by the amount of electrical current flowing between the negatively charged tool and the positively charged workpiece. The tool is normally designed to have a mirror image of the final surface of the workpiece.

Since the tooling, known as a cathode, never touches the workpiece, there is virtually no tool wear in the process. Typical deburring and polishing times are extremely fast — between 10 and 30 seconds for most applications. Depending on production requirements and the workpiece size, multiple part fixturing can be used to obtain high production rates.

The cathode that will generate the radius at the teeth-side face intersection could be of different design. Kennametal can produce simple ring cathodes or more complex, real counter profile segment cathodes following the exact contour with extremely limited impact on the side face. %%0815-KM-3%% shows the pro and cons of each solution.

ECM Gear Machining
Gear parts typically will be machined in the soft stage with an edge break of 0.2 mm up to 0.7 mm on all teeth-end sides. Cycle time is less than a minute and, as mentioned, multiple parts could be fixtures for efficient machining production. The results will be 100 percent free from particles and secondary burrs and will meet the most stringent quality requirements in terms of accuracy, repeatability, and surface finish.

Excellent productivity (short cycle times and multiple parts per cycle) in combination with the highest process stability and process controllability offers a cost-attractive solution to improved-quality gears.

Components of automatic transmission such as sun wheels, output shafts, center gears, guide wheel-shaft, and planet wheels are typically the parts on which we apply radiusing of the gear teeth. Typical application involves multiple fold fixtures to increase productivity, such as up to six satellite gears processed at the same time.

In industries facing large-volume production, the solution usually involves dual or multiple electrochemical stations with a built-in handling system between them combined with a robot as an interface with the production line. The electrolyte system including the filtration is sized accordingly to the production and the volume of material to be removed.

Process Benefits
Fast is perhaps the first characteristic of the ECM process. Shaping and radiusing the contour of the teeth of a satellite gear is not taking more than 7.5 seconds per part. In fact, Kennametal does six parts at the same time, the time cycle being 45 seconds. %%0815-KM-4%%

The first consequence is a huge positive impact on the cost per part. Combining shaping and finishing in a single operation also helps to limit the footprint dedicated to these two operations in the customer plant, which can also drive savings (less space, less conveyor and handling system, less different equipment to handle, driving less spare inventory, and less maintenance).

An additional customer benefit to high productivity is driven by the process stability, resulting in a higher consistency between parts and across the production as there is little to no cathode tool wear. This is a tremendous advantage against other processes, such as brushing, when it comes to ensuring a perfect control of the process and to matching the demanding accuracy both in terms of tolerances and shape.

As the equipment also proactively monitors and manages various parameters — such as power, voltage, pH, temperature, and electrolyte conditions — you are on a safe side. And if immediate remote assistance is needed, the equipment is also ready for online monitoring so an expert from Kennametal can connect to your machine and provide support. Kennametal can also analyze the running process and recommend adjustments to optimize the production.

Conclusion
The ECM process is mechanical and thermal load-free and will not generate any secondary burrs like a typical chamfering operation would. Also, it’s possible to add other ECM machining operations in other locations of the part. An example could be finishing of the edges of a key slot, contouring of a shaft pocket, polishing of a gear hole, and a marking operation, all at the same time of the gear teeth’s main contouring operation. 
 

A Discussion of Precision Electrochemical Machining (PECM)
Precision Electrochemical Machining (PECM) is a nonconventional machining process that can help deliver high-quality components. For specifics, we have asked Kennametal Precision Surface Solutions Product Manager Patrick Matt to explain more about the process.

Q: What is PECM?
A:
Precision electrochemical machining (PECM) is an electrochemical cavity-sinking erosion process with oscillating electrodes and a regulated working gap. It applies a pulsed direct current between the electrode and the workpiece. The workpiece dissolves anodically in accordance with the geometry of the subsequent electrode. This gives rise to complex geometrical shapes in practically all electrically conductive metals, e.g., in highly tempered steel, rolling bearing steel, powder-metallurgy steel, and super-alloys. PECM thereby also taps applications that could not be manufactured feasibly or at all in the past.
    Shaping, drilling, or micro-structuring on external or internal areas are typical applications PECM can achieve efficiently. The process is able to maintain tolerances ranging from 2 to 5 μm. Roughing, finishing, and polishing are typically combined into one operation. Achievable surface finish is 0.05 μm. Feed rate of cathode between 0.1 mm/min and 2 min/mm (application related). Typically, multiple parts are machined within one cycle to achieve an attractive cost per part.

Q: What is the background with this technology?
A:
Historically speaking, PECM technology first emerged in the 1990s and was exclusively associated with adding fine details to razor blade manufacturing. Machines appeared on the market in 2006, and for the past three years, Kennametal Precision Surface Solutions and PEMTEC became partners to provide PECM Centers to global regions, including China.

Q: What does Kennametal Precision Surface Solutions offer regarding PECM technology?
A:
Depending on the region, Kennametal Precision Surface Solutions offers customers equipment (machine and specific tooling and post-conditioning systems). Additionally, we offer process development support and subcontract activity to support either testing, ramp-up of production, or simple contract activities.

Q: What are the key features that drive the adoption of PECM?
A: Lower cost per part:

• Short cycle times due to multiple machined within one cycle (typically 4–60 pcs. per cycle)
• Almost no electrode wear, therefore, very attractive running costs
• Independent from material hardness
• Reduction of process steps: shaping, surface finishing (up to 0.05 μm), and deburring into one operation

    Freedom of design:

• Once a negative shape is created in the electrode, each formed part looks like the other
• Almost all shapes and features are able to machine economically by PECM, which aren’t by conventional machining

    Part performance:

• Parts are free from any tension as neither mechanical nor thermal stress load will be applied to the part, so part performance and fatigue strength of part increases
• Surface generated is free from any structure

 

Q: What are the benefits of using PECM versus conventional machining processes?
A:
Benefits include:

• For a reduction in process steps, PECM creates finished features like shapes and holes free from burrs in a single step
• Superior surface finish up to 0.05 μm beside 2 to 5 μm tolerance
• Parts are free from any oxide layers and free from any tension
• As the electrode does not touch the part, the process is not limited by material properties (hard, soft, rigid, etc.) and, accordingly, offer capabilities unreached by other processes
• The running costs are attractive

Efficient cycle times: multiple parts machined within one cycle

Q: What is the key production criterion and limits to leverage a successful PECM adoption?
A:
Considerations are:

• Intricate shape and thickness of the component favor PECM
• High production rate and huge volume production contribute heavily
• Scope of dimension as a maximum surface to be machined at once has to be within 100-cm2 areas

Q: What is the potential of PECM technology in the market?
A:
PECM has the ability to become more than a niche technology as it delivers precise products with almost no post-processing effort (deburring and finishing) independent from material hardness, which is the base for lower-cost part production compared to traditional machining. There is obviously a market demand for such technology for hard-to-machine materials as well as cost pressures forcing the market to nurture innovations. As most of the markets look for higher efficiency, we will see more and more engineered parts coming up (higher stress loaded, smaller, more precise, exotic material hard to machine, etc.). To produce these parts, PECM could become a key process as it ensures the economic competitiveness of their users in the global market.

For more information on Kennametal’s Precision Surface Solutions PECM Center of Excellence, contact k-hlzg.information@kennametal.com.

Next-generation electrochemical machining without stray attack
Kennametal Precision Surface Solutions is announcing Extrude Hone™ EVO, the next generation of electrochemical machining solutions featuring proprietary generator technology delivering from 3 kw to 100 kw of power depending on machine configuration.

“Additionally, EVO delivers another exclusive added value: electrochemical machining without stray machining attack,” said Bruno Boutantin, global marketing manager at Kennametal Precision Surface Solutions. “Customers will enjoy the highest surface finish quality for improved component performance.”

“We expect Kennametal Precision Surface Solutions to take responsibility for the overall process, both for services as well as pure product delivery – covering all services areas,” said Stephan Kiefer, senior manager at ZF Group Saarbrücken, for which Kennametal Extrude Hone is a supplier of ECM solutions for gear radiusing. “This is of paramount importance to us. It goes without saying that dependable on-time delivery is essential to our division.”

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is the global marketing manager of Kennametal Precision Surface Solutions — a company that delivers shaping, finishing capital, and contract shop solutions by manufacturing and using a complete range of Extrude Hone™ products. Boutantin has over 25 years of experience in machine tools, especially in the automotive and aerospace industry. With a team of product and business development managers, Boutantin ensures that PSS delivers customers with the solutions they need. For more information, contact Boutantin at bruno.boutantin@kennametal.com.