Commissioned to develop a parking brake for JCB backhoe loaders, one company entered some unexplored territory when developing its non-energising ball ramp models
Back in the 1940s when US-based Ausco Products invented ball ramp brakes, the original design – referred to as an energising ball ramp brake – used a servo action, which fed some of the brake’s output torque back into the actuation mechanism. The result was increased torque without increased pedal effort. While these older designs were innovative in their day and are still common in Europe and developing countries, the energising ball ramp brake is not without its problems. The brake is mechanically inefficient and, in some cases, difficult to control. Brake lock-up has always been a potential problem with this design, especially when used in dry brake applications.
In the 1990s, however, Ausco reinvented the ball ramp brake with a new style non-energising brake. The new brake increased mechanical efficiency from around 50% (for wet energising brakes) to around 75-80%, thereby reducing the pedal effort required. It also eliminated instability problems allowing for better control and eliminating lock-up potential. Moreover, the new non-energising design has proven to be more cost effective and is very successful in a wide variety of applications in the offhighway market.
When JCB began work on a new powershift transmission for its backhoe loaders and telehandlers, the company decided to add a parking brake directly to the transmission. Historically, these machines used mechanical caliper brakes for parking, yet JCB wanted to move to a mechanical, wet multidisc brake, which presented a number of challenges. Its specifications required that the brake produce adequate torque at a very low pull rod force with very limited cable travel. Because wet brakes have a lining coefficient of friction, which is about one-third that of a dry caliper brake, it is a difficult challenge to get enough torque with such limited stroke. Another constraint was the control of drag losses: as the brake is attached to the transmission, the brake spins at a very high speed, which created a potential for excessively high drag that had to be controlled. The company also wanted an ultra-high durability design, which would be robust enough to reliably handle the rough and tumble application of heavy-duty construction equipment. The brake also needed to be able to return the parking brake cable without the addition of external return springs.
Finally, European parking brake regulations require that a parking brake must be capable of stopping the machine from full speed/full load, while giving the operator the ability to modulate the brake during the stop.
Rising to the challenge
When JCB first approached Ausco with this application, the supplier decided that a non-energising ball ramp would be the way to go. An energising brake would be difficult in this application due to its low mechanical efficiency. To make up for the inefficiency, an energising brake would have to make more use of its servo feature to reliably boost the torque to the desired levels. This would make modulation and control difficult, and Ausco also decided that the brake simply would not meet cost objectives.
Instead, the company designed a non-energising disc brake for the application. The initial design used the same technology as previous brakes from the 1990s. However, in testing the unit, it was found that the brake had too much directional sensitivity. The torque in one direction varied by as much as 30% when compared with torque in the opposite direction. This directional sensitivity was due to friction feedback from the lining into the actuator. Ausco’s traditional solution to this problem is to coat the actuator with a special, hightemperature Teflon, which reduces the friction feedback into the actuator to acceptable levels. This solution was used successfully for more than 10 years on a variety of applications.
However, this application is different. JCB’s low pull force requirements created a very real problem. In this case, the total load on the actuator was so low that even with the Teflon coating, the friction feedback from the rotating disc became a considerable percentage of the total load on the actuator. Teflon coating it was just not enough to reduce the friction feedback to acceptable levels. Ausco had to find a cost-effective way to alleviate this effect. So it designed a roller needle thrust bearing to prevent friction feedback and virtually eliminated the directional sensitivity issue. The brake’s torque in the forward direction is within 2% of the torque in the reverse direction. It also increased mechanical efficiency to as high as 95%. This means that the brake has no trouble meeting the torque requirements, even with the ultra-low pull forces and travel restrictions. In addition, the needle bearing design proved very cost effective. Because the bearing has a safety factor of close to 50:1, virtually any bearing source can be used. The loading is also low enough that it effectively prevents any damage to the mating surfaces, thereby eliminating any need for thrust washers or special surface-hardening treatments.
The brake can spin at speeds in excess of 3,500rpm, so control of drag was an absolute necessity to prevent huge potential power losses. There are two primary keys to control of brake drag. The first is related to control of sump level – as the discs spin in the oil, the oil is continuously sheared by the discs, and as the sump level increases, the amount of shear increases dramatically. The second key is running clearance – if the brake discs do not have adequate running clearance, incidental contact of the lining with the mating surface increases, and so the tighter clearances increase oil shear. The result is excessive drag and large power loss.
To control sump level, JCB designed a central oil-feed system for the brake. The oil feeds into the centre of the brake from the transmission, and the brake housing then drains the oil back to the transmission to prevent overfill.
To control running clearance, Ausco made use of the needle bearing. Normally, the challenge of controlling running clearance in a mechanical brake is a balancing act between running clearance and mechanical advantage. As the brake’s mechanical advantage goes up, the allowable running clearance goes down. However, since the needle bearing increased mechanical efficiency, the need for a high mechanical advantage was reduced. The final result was that Ausco had no problem obtaining sufficient running clearance to prevent excess drag without affecting the brake’s primary function.
Historically, ball ramp brakes have used extension springs to return the actuator – they do not return the cable. With the JCB application, however, the springs needed to pull the cable back as well, so Ausco carried out considerable testing on a variety of spring designs.
What the company found was that it is difficult to get a spring that fits, while also having enough force to apply sufficient load on the cable to get a reliable return. However, once the needle bearing was put on the brake, a new possibility opened up. The needle bearing reduces the friction of the actuator to a point at which an axial compression spring became possible. The axially placed spring is actually working against the mechanical advantage of the brake. In earlier designs, such a spring was unreliable due to the friction in the actuation mechanism. With the needle bearing design, the spring proved to be simple, inexpensive and highly effective.
JCB and Ausco put a lot of test and development time into a new patent-pending brake. The brake passed all laboratory and field testing and went into production in 2004. Like any new product, the real test is not in the lab – it is in the field, and so far, the brake has not had a single warranty or customer complaint. The design has proven itself to be completely effective and troublefree in this application. For an item as important as a brake, it does not get much better than that.
Brian P. Dennis, P.E. is sales and design engineering manager at Ausco Products, Inc, in Michigan, USA