93 Steering linkage ball and socket joints

All steering linkage layouts are comprised of rods and arms joined together by ball joints. The ball joints enable track rods, drag-link rods and relay rods to swivel in both the horizontal and vertical planes relative to the steering arms to which they are attached. Most ball joints are designed to tilt from the perpendicular through an inclined angle of up to 20° for the axle beam type front suspension, and as much as 30° in certain independent front suspension steering systems.

The basic ball joint is comprised of a ball mounted in a socket housing. The ball pin profile can be divided into three sections; at one end the pin is parallel and threaded, the middle section is tapered and the opposite end section is spherically shaped. The tapered middle section of the pin fits into a similarly shaped hole made at one end of the steering arm so that when the pin is drawn into the hole by the threaded nut the pin becomes wedged.

The spherical end of the ball is sandwiched between two half hemispherical socket sets which may be positioned at right angles to the pin's axis (Fig. 9.24(a and b)). Alternatively, a more popular arrangement is to have the two half sockets located axially to the ball pin's axis, that is, one above the other (Fig. 9.24(c-f)).

The ball pins are made from steel which when heat treated provide an exceptionally strong tough core with a glass hard surface finish. These properties are achieved for normal manual steering applications from forged case-hardened carbon (0.15%) manganese (0.8%) steel, or for heavy duty power steering durability from forged induction hardened 3% nickel 1% chromium steel. For the socket housing which might also form one of the half socket seats, forged induction hardened steels such as a 0.35% carbon manganese 1.5% steel can be used. A 1.2% nickel 0.5% chromium steel can be used for medium and heavy heavy duty applications.

Modern medium and heavy duty ball and socket joints may use the ball housing itself as the half socket formed around the neck of the ball pin. The other half socket which bears against the ball end of the ball pin is generally made from oil impregnated sintered iron (Fig. 9.24(c)); another type designed for automatic chassis lubrication, an induction hardened pressed steel half socket, is employed (Fig. 9.24(d)). Both cases are spring loaded to ensure positive contact with the ball at all times. A helical (slot) groove machined across the shoulder of the ball ensures that the housing half socket and ball top face is always adequately lubricated and at the same time provides a bypass passage to prevent pressurization within the joint.

Ball and socket joints for light and medium duty To reduce the risk of binding or seizure and to improve the smooth movement of the ball when it swivels, particularly if the dust cover is damaged and the joint becomes dry, non-metallic sockets are preferable. These may be made from moulded nylon and for some applications the nylon may be impregnated with molybdenum di-sulphide. Polyurethane and Teflon have also been utilized as a socket material to some extent. With the nylon sockets (Fig. 9.24(e)) the ball pin throat half socket and the retainer cap is a press fit in the bore of the housing end float. The coil spring accommodates initial settling of the nylon and subsequent wear and the retainer cap is held in position by spinning over a lip on the housing. To prevent the spring loaded half socket from rotating with the ball, two shallow tongues on the insert half socket engage with slots in the floating half socket. These ball joints are suitable for light and medium duty and for normal road working conditions have an exceptionally longer service life.

For a more precise adjustment of the ball and socket joint, the end half socket may be positioned by a threaded retainer cap (Fig. 9.24(f)) which is screwed against the ball until all the play has been taken up. The cap is then locked in position by crimping the entrance of the ball bore. A Belleville spring is positioned between the half socket and the screw retainer cap to preload the joint and compress the nylon.

9.3.3 Ball joint dust cover (Fig. 9.24(c-f)) An important feature for a ball type joint is its dust cover, often referred to as the boot or rubber gaiter, but usually made from either polyurethane or nitrile rubber mouldings, since both these materials have a high resistance to attack by ozone and do not tend to crack or to become hard and brittle at low temperature. The purpose of the dust cover is

Fig. 9.24(a-f) Steering ball unit

to exclude road dirt moisture and water, which if permitted to enter the joint would embed itself between the ball and socket rubbing surfaces. The consequence of moisture entering the working section of the joint is that when the air temperature drops the moisture condenses and floods the upper part of the joint. If salt products and grit are sprayed up from the road, corrosion and a mild grinding action might result which could quickly erode the glass finish of the ball and socket surfaces. This is then followed by the pitting of the spherical surfaces and a wear rate which will rapidly increase as the clearance between the rubbing faces becomes larger.

Slackness within the ball joint will cause wheel oscillation (shimmy), lack of steering response, excessive tyre wear and harsh or notchy steering feel.

Alternatively, the combination of grease, grit, water and salts may produce a solid compound which is liable to seize or at least stiffen the relative angular movement of the ball and socket joint, resulting in steering wander.

The dust boot must give complete protection against exposure from the road but not so good that air and the old grease cannot be expelled when the joint is recharged, particularly if the grease is pumped into the joint at high pressure, otherwise the boot will burst or it may be forced off its seat so that the ball and socket will become exposed to the surroundings.

The angular rotation of the ball joint, which might amount to 40° or even more, must be accommodated. Therefore, to permit relative rotation to take place between the ball pin and the dust cover, the boot makes a loose fit over the ball pin and is restrained from moving axially by the steering arm and ball pin shoulder while a steel ring is moulded into the dust cover to prevent the mouth of the boot around the pin spreading out (Fig. 9.24(c-f)). In contrast, the dust cover makes a tight fit over the large diameter socket housing by a steel band which tightly grips the boot.

9.3.4 Ball joint lubrication

Before dust covers were fitted, ball joints needed to be greased at least every 1600 kilometres (1000 miles). The advent of dust covers to protect the joint against dirt and water enabled the grease recharging intervals to be extended to 160 000 kilometres (10 000 miles). With further improvements in socket materials, ball joint design and the choice of lubricant the intervals between greasing can be extended up to 50000 kilometres (30000 miles) under normal road working conditions. With the demand for more positive and reliable steering, joint lubrication and the inconvenience of periodic off the road time, automatic chassis lubrication systems via plastic pipes have become very popular for heavy commercial vehicles so that a slow but steady displacement of grease through the ball joint system takes place. The introduction to split socket mouldings made from non-metallic materials has enabled a range of light and medium duty ball and socket joints to be developed so that they are grease packed for life. They therefore require no further lubrication provided that the boot cover is a good fit over the socket housing and it does not become damaged in any way.

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