This cutaway model of a spring brake actuator shows the compression spring on the right and the diaphragm service brake on the left.

Sprint brake actuators will, under the build up of pressure, release a bus or truck’s mission for the day. Many truck owners will recount an ‘accidental mission’ when a vehicle is parked on a slope and the driver releases the handbrake prior to correct air-pressure buildup. With the engine idling while visiting the toilet or some admin activity outside of the driver’s seat, the pressure slowly builds to a release point – around 8,5bar - and ‘Eishhhhhh! Die lorrie het op sy eie weggehol!’ More attention needs to be paid to ‘spring brake’ systems writes FleetWatch technical orrespondent Dave Scott.

A spring brake actuator is a standard component of an airbrake system – air/ hydraulic or full air. But 40% of South Africa’s trucks are under 8 500kg gross vehicle mass (GVM) – the medium class segment – and here you will not find spring brake actuators because medium trucks operate on vacuum/hydraulic braking systems. All trailers over 3 500kg GVM are air braked and equipped with spring brake actuators. 

Loose terminology makes for confusion

Many descriptions are attached to the complete airbrake actuator and spring brake unit – ‘boosters’, ‘emergency brakes’, ‘spring brakes’ and because they operate during long periods of parking the vehicle, they also get incorrectly named as ‘emergency park brakes’. If the system has a leak, the pressure can decrease enough to engage the spring (emergency) brake. Park brake is on a protected circuit, the whole concept being that should there be total loss of air in the park brake that the spring brakes will act automatically as emergency brakes. The complete assembly consists of a single-diaphragm air service brake in the front section with the spring brake actuator in the rear section. Note that air pressure is reduced to apply a spring brake actuator while pressure is increased to apply the air service brake actuator. 

From an internet search it appears that the correct technical description is a spring brake actuator and because of emergency characteristics, it’s only fitted in matching pairs to the rear axles of any unit. 

Training & safety rules are essential 

A spring brake actuator uses very powerful potential energy stored in a compression spring to apply a wheel’s foundation brakes. This means that when driving the spring must be maintained fully compressed so that brakes stay released. And even when there is no air in the system, a spring brake is still under extreme compression, a fact that ignorant maintenance crews can attest to with severe injury and loss of life. Releasing a compressed spring from its chamber will take off someone’s head, if you’re in the spring’s path as it accidently explodes outwards. The only way to disassemble a spring brake actuator is in a safety cage and never at the side of the road. 

And environmentally, disposing of spring brake actuator units is a problem. They should be disassembled in safety – inside the cage – before being discarded as scrap as inside there’s a coiled spring under compression waiting to escape and bite somebody. All training and specification manuals contain the following warning: 

It’s all about the correct torque

Without correct mounting bolt torques, any spring brake non-pressure housing is prone to cracking due to vibration. WABCO Automotive SA specifies that mounting bolts be tightened to 210 Nm torque per mounting bolt and failure to observe this is where the whole ‘booster’ unit is seen to rip out and fall off. Why? It’s simply because most workshops are not equipped with a maximum-rated 350Nm torque wrench and extension piece – and the task cannot be carried out without the extension fitting as there’s just not enough space in there to tighten down the mounting bolts with the torque wrench alone. So spring brake actuators are mostly fastened by feel and that’s just not enough apart from being too variable. 

What’s the life of a complete spring brake and single diaphragm unit? It’s very operational and route dependent, but in terms of time, a minimum of 12 months or one million cycles seems to be the average unit life before replacement. Then fatigue sets in and seals start to leak – that’s a problem that leads to unnecessary expensive brake valve replacements when the actuator unit is the real leaker. 

Essential to fastening 'boosters' to 210Nm is this max rated 350Nm torque wrench and extention piece - without this the 'booster' will fail.	A spring brake in uncompressed state. This is what rockets out of its chamber if released in ignorance and can mortally wound a person.

This is the result of insufficient torque being applied to mounting bolts - torn boosters.	Common malpractice is the fitment of different size spring brake boosters to the same axle causing uneven braking forces.

A huge market potential 

How big is the spring brake actuator replacement market? The current registered truck, bus and trailer population looks like this:

Taking 40% off the truck parc for medium trucks that are not equipped with spring brakes – 40% off 318,000 = 127,200 – leaves a potential market of 378,200 vehicles every year that could fit at least two spring brake actuator units.

That’s a staggering total market potential of 756,400 spring brake actuators per annum! This is a conservative figure as no account is being taken of 6 X 4 trucks tractors as well as tri-axle and double-axle trailers. No wonder prices rules and everyone is chasing it. The numbers also point to an environmental mess of braking components if one tops this up with discarded brake drums. 

Lack of standards 

FleetWatch’s Brake & Tyre Watch projects around South Africa reveal the extent of ignorance around spring brake actuators. Loose clamps, chafing hoses, loose ‘boosters’ and incorrectly connected pushrods to slack adjusters are just some of the problems encountered. Worse still, different size spring brake actuators are fitted on the same axle when they are clearly providing unequal pressure to either wheel on the same axle. 

Enoch Silcock, National Sales and Marketing Manager at WABCO SA, is concerned over the lack of standards on this component. “The biggest cause of in-service spring brake unit failures is lack of adherence to the required torque for mounting bolts. Everyone assures us they are fastening to 210Nm but very few can produce a calibrated torque wrench with the essential extension piece.” 

A lack of standards for spring brake actuators makes this a conflict zone for safety, price and quality. Enoch Silcock Wabco Sales & Marketing Manager

Short & long stroke spring brake actuators (boosters) 

On spring brakes a type number indicates the unit size. Most boosters fitted in South Africa go by the numerical code of Type 16/20 or Type 20/24, 24/30 or even Type 30/30. The first two digits refers to the size of the service brake chamber while the second pair refer to the emergency spring brake chamber size. Wolfgang Lehmann of ITI Services points out – “Different size spring brake boosters on the same axle will result in uneven breaking forces – this means either over or under-braking. It’s very important that similar size units are fitted to the same axle." 

Lehmann adds – “Long-stroke boosters are increasingly fitted to trailers running within the 56 ton gross combination mass (GCM) limit because the greater stroke rate provides a high pushrod force and higher stroke margin at or near the recommended re-adjust position. 56 ton vehicle combinations need the help they can get in terms of braking capacity and improving braking performance when brakes heat up and drums expand.” When it comes to 6 –axle articulated rigs now operating close to 50 ton GCM, it’s necessary to fit spring brake actuators (boosters) to all three trailer axles and not just the second and last axle only. Having all three axles equipped with the correct size boosters would match Compulsory Standards for Park Brake performance.” 

MBWA must become an entrenched culture

In road transport the practice of Management By Walking Around – MBWA – must be part of the daily business culture. This is when mismatched ‘booster’ units are seen on the same axle while pushrods of unequal length are incorrectly connected to slack adjusters – you don’t have to be very qualified to spot this botch-up. ‘If managers and supervisors don’t care, why should we?’ That’s also an entrenched culture of some operations. 

And then take an integrated view of brakes. What about the disconnected or incorrectly set load sensing valve? How good are we at air-leak detection? Remember that air leaks cost fuel through excessive idling. 

What’s our purchasing policy and laid down standards for spring brake actuators – the market is flooded with cheap units – do they last – are we only tracking costs and not incidents? FleetWatch’s Brake & Tyre Watch projects show that this area of braking is out of control. Unfortunately the consequences are loss of life and income. Spring brake actuator failure can be the kick-start to a cascade of events that leads to a major road accident. 
 

Braking Standards? Yeah sure! The state of this booster and slack adjustor tells that brakes don’t even feature in the thoughts of this operator who found his truck taken off the road in one of the FleetWatch Brake & Tyre Watch projects.	MBWA – Management By Walking Around easily spots shocking attention to braking standards. Here we see service brake actuators trying to work with badly connected slack adjusters of unequal length in any hole available. It’s sloppiness at its best!