A Brief Summary of Electric Vehicle Axles and Drives
By Jim Criscuolo, General Manager, Oerlikon Graziano - North America
John Strickland, Vice President of Marketing and Strategic Planning, Oerlikon Fairfield
Commercial Products which use electric motors and drives, range from systems used to propel automated guided vehicles, wheelchair and other personal mobility products, electric drives for diesel-electric locomotives, up to electric drives for large off highway dump trucks as well as marine drives for ferry and cruise ships.
For the purpose of this article we will focus on the products that serve the golf, utility and turf vehicles, material handling axles and drive systems, automotive and light-medium truck applications as well as some of the larger vehicles that should be of interest to the engineer in all of us.
General Design Considerations:
In most pure electric vehicles, the noise generated by the electric motor is a fraction of that produced by an internal combustion engine. As a result the gear train cannot count on its noise level being hidden behind the exhaust or valve train noise of its power source. A great deal of attention is paid to the design and manufacturing methods to ensure a quiet product. The interaction of gear tooth module, helix angle, face width and contact ratio need to be optimized to carry the load as well as produce a low level of transmission error which results in noise. In many cases ground gears are the solution to noise issues, but in more cost sensitive applications hobbed and shaved gears can achieve excellent and reliable results. The process requires reviewing the final geometry after heat treatment and making adjustments to the shaving operation to dial in the contact pattern to produce a low noise result.
In the case of DC motors the ability to produce very high torques at locked rotor and low RPM means that the gear train needs to be able to absorb significant shock loads. High quality gear steels and appropriate carburizing with adequate case depth is necessary to avoid pitting during high stress load. In all applications, lubrication is an important factor in design. With the introduction of AC motors and their ability to push the speed envelope from under 6,000 RPM to over 10,000 RPM, this issue is even more critical. In the development process of a new gear drive axle, a functional clear plastic case is often built up and tested to allow engineers to see the oil flow during operation. Each bearing is checked at a variety of RPMs and tilt angles. The goal is to be sure that all components receive adequate lubrication throughout the operational envelope.
The vast majority of electric drive axles are single ratio with no opportunity to shift either into a higher gear or reverse. The RPM flexibility inherent in the electric motor and its ability to operate in either direction makes the gearbox task much simpler than the comparable device that would be attached to an internal combustion engine. As a result more efficient and compact designs are possible. In some unique applications where the operating speed range is very high a two speed electric drive gearbox can offer the necessary operational envelope. One application for this is high speed and high performance electric automobiles, which require the low end torque for acceleration combined with the ability to reach high cruise speeds.
Golf Axles:
- High Volume Fleet Axles
With industry wide volumes exceeding 150,000 units annually this is one of the largest volume electric vehicle applications in the world. The designs in the market have proven to be reliable and versatile. The cost reduction and manufacturing efficiency efforts focused on these products have made them a great value for OEM industry as well as a source of low cost components and sub assemblies to incorporate into related products. Products used in the Golf industry have a GAW rating in the 1600-2060 Lb (727-934 KG) range while the utility and turf market may call for higher capacities up to 2600 Lb (1182 KG) GAW. The product which Oerlikon Graziano makes for Club Car Ingersoll Rand is a robust design capable of serving all applications from Fleet Golf up to the extended frame personnel and burden carriers. Other approaches to the range of application requirements include having a light duty product focused on golf and a heavier duty model for utility purposes. OEM’s select the breakdown that best suits their particular situation.
- Individual Cars
The golf car for personal use can range from a fleet car with no modifications, all the way to a custom bodied car which might resemble a sports car or SUV and be capable of speeds in the range of 20 MPH. For the axle this usually means modification of the drive ratio from the standard (approximately 12:1) to a higher speed 10:1 or even lower. While this can reduce hill climbing ability the use of a higher output motor and controller can overcome this drawback.
- NEV – LSV product Drives
In addition to the ratio changes above the Neighborhood Electric Vehicle which falls under the FMVSS for a Low Speed Vehicle has a number of other safety and performance modifications. Although it is not required by FMVSS, many of these vehicles adopt a wider track than the golf car standard. This results in the hub-to-hub width of the axle expanding from the golf standard around 38 inches to 40-42 inches or more. The adoption of independent rear or front wheel drive allows a fixed center section to be used. This is often based on the common golf axle components with splined stub outputs that connect to CV-type half shafts. The additional width can be accommodated by these shafts.
- AC Drive Considerations
The advent of AC motor in the Golf and related markets has brought the following major advances:
- Higher RPM potential due to the lack of limit caused by the brushes and commutator on a DC motor.
- Improved dynamic or regenerative braking using the motor to slow or control the vehicle down to nearly zero speed.
- Improved efficiency and control over the performance characteristics of the operating envelope.
What this means for the axle is that it is now being operated at higher input RPM and being asked to transmit torque both to and from the wheels. While this is not a major concern for a properly designed drive train, some attention to the high speed lubrication and torque reversals needs be paid. Depending on the environment and the aggressive nature of the dynamic braking it may be necessary to consider the potential for single wheel slip during braking. The incorporation of a limited slip or locking differential may be necessary to ensure the minimum stopping distances possible on slick surfaces.
Utility Vehicle Axles:
- GVW considerations
When vehicle and axle loads exceed the capacity for the axle products based on the golf car design, a new product range comes into play. In the beginning utility vehicle manufacturers pulled automotive rear drive axles from recyclers (they were called junk yards at the time). They cut down the length of the axle shafts and tubes and replaced the input yoke with a pulley or sprocket. Then they attached a bracket and motor with a matching pulley or sprocket on top of the center section. The resulting belt or chain drive combines with the ratio of the rear axle itself, gave an overall ratio of 9 to 15:1. The chain/belt drives were noisy and maintenance prone and eventually the supply or rear drive axles dropped off with the switch to front drive in most cars. As a result, Graziano and Taylor-Dunn collaborated to produce a “built-from-scratch” replacement with additional features. The final product includes a 3 shaft helical drop box combined with a spiral bevel final drive. The middle gear of the helical drop is an idler that runs on an eccentric cantilevered shaft mount. By exchanging the input and output helical gears and adjusting the eccentric to put the idler in proper mesh between them, 5 overall ratios can be achieved between 12 and 30:1.
The result is a product that is rated to 6830 lbs (3100 KG) GVW and can accept open bell DC motors of the type used in the golf market as well as AC drives. It can develop low speed torque capable of towing and carrying significant loads as well as achieve speeds in excess of 20MPH. It is used in a wide variety of Taylor Dun Products including Burden Carriers and Personnel Carriers.
Turf Vehicle Axles:
Historically I/C Engines have been the dominant power source for Turf products, but non slip differential offerings have expanded the mission for electric drives.
The development and incorporation of a non-slip differential into the Electric Golf Axle product has opened up the envelope of where this product can be used effectively. Through the efforts of Club Car Service Parts, Hilliard Corporation has developed a drop in replacement for the Graziano differential housing. It is packaged with the appropriate oil and fasteners and available through Club Car Service. After market rebuilders and small OEMs are looking at this product to open up the use of electric vehicles to the off road recreational user.
- Potential for 4WD with 2 low cost electric drives
The relatively low cost of a transaxle and motor when compared to a similar I/C package make it possible to consider a 4WD powertrain with 2 separate drives, one front and one rear. Since the 2 drives are not mechanically linked they will not bind up and fight each other as long as they are getting the same or similar signals for speed and acceleration. This approach can include a live axle in the rear and an Independent front suspension in the front for steering. Both drives can take the same motor and have the same gear ratio.
Material Handling Drives:
- Electric Driven Fork Lifts
There are many electric driven fork lift trucks in the market. Features vary, but the core mission is still to transmit torque to drive the vehicle, support and carry the load as well as stop the load with precise control. For their customer Nissan in Spain, Oerlikon Graziano provides an axle which uses parallel shafting and a transverse motor mount built into the cast iron housing of the axle. There is also a version with an in-line shaft input for a motor mounted on the chassis of the truck. This allows the weight of the motor to be shifted rearwards in the vehicle to reduce the requirement for counter balance.
The materials handling industry lead the way in electric power with electric / battery powered industrial forklifts used inside plants and factories. Oerlikon’s Drive Systems group participates in this industry with Graziano providing electric drive axles and differentials for several leading lift truck producers, and Fairfield providing independent electric wheel drives for both industrial lift trucks, automated wire guided vehicles (AGV), as well as for electric powered personnel lifts. In the early 2000 era Fairfield designed and produced a self contained electric “steer – able” wheel drive which included a brake and integrated electric DC motor. This product was targeted at the industrial scissors lift market and replaced more commonly used hydraulic wheel motors that were powered by electric motors. The Fairfield electric drive has been proven to be more efficient, cleaner, and compact than the traditional hydraulic – over – electric propel system.
Automotive Drives:
- Micro Light Commuter Vehicles
Golf Axle Applications: In applications that fall under 2600 Lb GVW versions of the golf type transaxle are being applied. In some cases the application is FWD and requires only the center section with output shafts for universal or CV joints and half shafts. Light Bodies and Frame allow these vehicles to operate with many of the comforts of a small gas powered automobile while maintaining the efficiency and cost of a light electric powertrain. Feel Good Cars uses Graziano Independent suspension version of the golf axle to drive their line of Electric Front Wheel Drive Products.
- Light CommuterVehicles
Transaxle: Oerlikon Graziano has been working to develop a front wheel drive axle that meets on highway requirements for a small electric car. This development has had it’s stops and starts but is now headed into production with Th!nk Nordic. The axle is designed for AC motor drive and is capable of operation above 10,000 RPM with an output torque of 900NM. The basic layout is similar to the golf axle center section with 3 shafts and a ratio of about 10:1; however it is much stronger and has additional sensor features as well as a locking pawl for park brake.
Light and Medium Truck/Bus Drives:
- Light Trucks
- Utility Axle Applications
The Utility transaxle produced initially for Taylor Dunn has seen applications in light truck conversions to electric and hybrid drives. The wide choice of ratios and the robustness of the cast iron housing make it suitable for this extended role.
Hybrid Conversions
Oerlikon Graziano has been endeavoring to become involved in this market where production trucks and buses are converted to a hybrid electric drive. In these cases, the existing engine and rear axle are left in place, while a generator, batteries, controller and motor are installed in place of the original transmission. The gearbox is used to adapt the motor output to the RPM requirements of the final axle drive. The technological players in this market at Enova Systems are experts in all aspects of this drive system, but their core competency is in the electronics and controls. It is in the design and manufacture of the gearbox that Oerlikon Graziano can bring its core competency to bear.
Hybrid Transmissions
In this application we are talking about a transmission that is capable of transferring torque from the I/C engine to the final drive while simultaneously adding power from the batteries or extracting power to recharge them. The Transmission consists of a complete automatic transmission, intimately combined, in the same case, with a series of motor/generators. Needless to say the controller logic and circuitry is what makes all this possible. Oerlikon Graziano is responsible for providing the very demanding gear sets that are used by Allison Transmission in their EV Drive products. The application is in Buses where the stop and go duty cycle can be translated into a great deal of recaptured energy that can be reused to provide critical acceleration.
Electric Drives for Off – Highway and Construction Machinery Applications
Various types of construction machinery use electric drive technology for both propel and work functioning. The two most critical factors influencing the shift to electric drive technology include the increasingly high cost of fuel (both diesel and other petroleum products), and the global influence toward “green” sources of power as they tend to offer a cleaner, more environmentally friendly alternative.
In addition to the materials and personnel lift markets, Fairfield has been involved in the electric and propel drives for other off – highway applications as well, such as producing gears and drive components for large multi – ton haul trucks used in mining and quarry haulage applications. These gears and drive components become part of a high horsepower propulsion system powered by large AC drive electric motors. Fairfield has participated in this specific market for over a quarter of a century providing large planet gears, ring gears and gear carrier assemblies. Fairfield also has a long history providing large gears and drive systems for electric powered locomotive rail applications as well as personnel carriers and public transportation systems using both AC and DC electric drive technology. The electric drive market segment is an important area for Oerlikon’s Drive Systems Group, and both Graziano and Fairfield have a long, successful history participating in this industry.
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