- Indispensable: The component, which has been used billions of times over, will still be necessary as long as cars have wheels
- Tradition-laden: Lemförder patent 60 years ago ushers in era of maintenance-free ball joints
- Advanced: Thanks to material and process improvements, ZF is making ball joints more robust, less prone to wear, and lighter, while combining them intelligently with electronics
Wheels, brakes, springs, shock absorbers – and sometimes even the control arms, which form part of the wheel suspension and allow the wheel to turn, flex, and guide: Drivers do not normally get to see much more than these major components when it comes to their car’s chassis. Yet even in the very latest designs, a small component that is hidden away carries out an equally supporting role: ball joints. They provide a moving connection between the control arms and the wheel carrier – making it possible for the front wheels, for instance, to be turned in, while at the same time also safely transferring vertical spring movements. Ball joints have been carrying out these tasks since 1954 without any need for maintenance. Thanks to very latest technologies, ZF is also configuring them to be increasingly more robust and lighter.
Photo: ZF
The basic design of a ball joint resembles a human shoulder joint: The ball joints used in passenger car chassis comprise a ball stud made out of steel that sits backlash-free in a lubricated plastic bearing cup, which is, in turn, embedded in a steel or aluminum housing. A rubber sealing boot on top prevents dirt and moisture ingress as well as lubricant leaks. 60 years ago, the former Lemförder Metallwarengesellschaft m.b.H. registered this design with the German Patent Office – to quote the exact wording as a “joint, in particular for steering linkages for motor-powered vehicles.”
In 1984, ZF Friedrichshafen AG acquired 51 percent of the shares in the Lemförder group of companies, taking over 100 percent in 2003 – and with ZF Lemförder GmbH finally being fully incorporated into ZF Friedrichshafen AG in August 2011. Thanks to ongoing further developments, the technology company integrated a host of other useful features into the ball joints. Thus, the maintenance-free ball joints, which have been used in their billions, are today more than ever a globally successful product that enjoys strong demand: In 2013 alone, around 220 million units came off the ZF production line at 17 production locations in 12 countries – a new annual record.
Robust all-round talent
The current test cycles at ZF illustrate just how durable a ball joint in passenger car chassis – from subcompact cars to vans – has to be: The ball joint only gets the go-ahead for usage in volume production vehicles for instance when the relevant test component has previously withstood around a million test bench load changes without any problems. Similarly exacting requirements apply to the other criteria: For instance, the joint must not leak if it is subjected directly to 80-bar pressure washers. Furthermore, even short-term peak temperatures of minus 40 and plus 140 degrees Celsius must not impair its function. ZF designs every ball joint to withstand the rigors encountered throughout its entire service life.
The ball stud inside is extremely important since it is one of the components in the ball joint that is subjected to the greatest loads. In the case of a full-size high-end luxury car on the move it has to support around 2,000 kilograms – and do so over an area of just 2.4 square centimeters that connects it to the ball cup in this case. The forces exerted can double quickly if the vehicle happens to drive over the curb. And even then the movement of the wheel – for flexing and steering – must not suffer. At the same time, the ball stud must be strong enough to simply bend rather than break in response to enormous crash loads.
And to compound matters further, the requirements in terms of installation space and weight are also increasing – ultimately lightweight construction is one of the key criteria for reducing fuel consumption and, in turn, CO2 emissions. Altogether even very small components can achieve a great deal; even more so when up to 20 ball joints are fitted to a passenger car with a sophisticated multilink suspension. Therefore, and given they reduce the unsprung masses in the chassis, the robust ZF ball studs now weigh on average just 200 grams.
Progress through production expertise
ZF is improving both the characteristics and environmental footprint of the ball studs with new manufacturing processes. The most recent innovation is a manufacturing process which no longer requires any time-consuming, energy-intensive, and costly heat-treatment and coating processes: With so-called cold extrusion, ZF is using a new special steel whose material properties prevent corrosion.
ZF has been using another resource-saving process for a long time: Pressing stud blanks as near as possible to the finished-part contour (near net shape) means only a few grams of steel need to be machined off. And even this “waste material” is then fed back into the steel manufacturing process.
Ready for the future
Other innovations not only relate to the hardware: With the integrated sensor module, intelligent electronics are also finding their way into the very latest evolution stage of the ball joint. Thus, this setup manages for the first time to record information on the driving condition directly in the ball joint. This information may provide a valuable additional source of data for various assistance systems such as the Park Distance Control.
Yet regardless of their specific configuration and irrespective of whether cars are powered conventionally or using an electric motor, it is clear that the maintenance-free ZF ball joints will remain indispensable in the chassis.
