EEC braking standards pose problems...
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;pecially for trailer iufacturers and uerters when it comes to isuring response time. A II Essex company of ineering consultants has ?loped a compact, able tester to solve the flem.
■ E the subject of EEC braking idards with almost any lp of transport engineers you can be sure of starting a y discussion which could inue until the Common Agriiral Policy cows come home. /e argument would almost ainly centre around the Alan whether the standards raking performance of comial vehicles in the UK have 1 improved or not by the )tion of EEC Directives.
ie concurrent introduction of )nal type approval for goods cies and the uplift in maxiri weights, all part of the harisation process, has tended ivershadow the recent nges in braking requirets, but now the EEC stenIs are well and truly with us. le Motor Vehicles (Construcand Use) (Amendment) (No. legulations 1981, SI 1189, :h was published in August I, requires all commercial cies (vans, trucks, trailers, and coaches) manufacd after October 1, 1992, and used after April 1, 1983, to ;fy the construction, fitting performance standards :ified in the EEC Directives l20, 75/524 and 79/489.
le amended C and U Regula5 also require every in-scope cle to be maintained so that applicable EEC braking reements are satisfied at all is when the vehicle is in use ie road.
is latter requirement is pararly controversial at present number of reasons.
)me engineers will argue new vehicles should have ler standards to allow for the itable deterioration that will Jr in service. Others will it to the glaring anomaly that ts as regards brake friction enals supplied as original pment compared with those Dlied in the after market.
lere are no standards EEC, or otherwise, for replaceit friction material, and Auto
motive Engineer recently reported the disturbing fact that a Unipart study of the quality and friction of materials from 171 suppliers worldwide revealed that fewer than 10 per cent had adequate facilities for testing and quality control.
It is not yet clear exactly how deeply involved fleet engineers will need to become in the considerable technical detail of the EEC/ECE requirements (now also C and U requirements).
Many smaller manufacturers and converters have already discovered that it is more economical to employ specialists when it Comes to plotting adhesion utilisation curves to show the distribution of braking effort among axles, for example, rather than attempt all the calculations themselves.
One such specialist is Century Technology automotive design and research engineers of Braintree, Essex. The two partners in this company, are both ex-Volvo engineers who decided to move into general automotive engineering consultancy.
Their work includes stress calculations and design studies, mainly for trailers or trailer conversions but also for some vehicle manufacturers.
Recently they have found that an increasing part of their work involves computer predictions for brake performance to satisfy the new regulations. They have an Apple micro-computer with a program specifically for this, When you begin to study Annex 10 to ECE Regulation No. 13 and EEC Directive 75/524 it soon becomes apparent why computers have become almost essential tools for this work.
Carefully defined limits are laid down in the regulations for the distribution of braking effort among vehicle axles, including trailers and semi-trailers, which although not yet subject to type approval in the UK do have to comply with the amended C and U Regulations.
The limits are defined in diagrams which show the relationship between braking ratio, which is defined as "sum of braking forces at periphery of wheels" over "total normal static reaction of road surface on wheels" usually called simply braking efficiency, and serviceline pressure.
It is this requirement which has made anti-lock braking or load sensing essential on vehicles and trailers.
One laudable objective is to prevent premature lock-up of the drive-axle wheels of tractive units, and thus prevent jack-knifing, in any load condition. Our brake testing of modern European tractive units suggest that by and large in practice that objective is not being achieved. But that is another story.
One of the problems facing trailer manufacturers and converters who until the introduction of the amended C and U Regulations did not have to concern themselves with such detail is that the regulations state that the "requirement shall be met for all permissible load conditions of the semi-trailer axles". So change one variable slightly and a whole new set of calculations is required. Which is where companies such as Century Technology come in.
But there is another requirement in both the ECE Regulation and EEC Directive which has caused even more head scratching among small manufacturers and it is one with which fleet engineers might find themselves having to grapple in the future. This is what the ECE calls "res ponse time" and what the EEC refers to as "reaction time" but which are in fact the same thing.
The requirement for powerdriven vehicles with compressed air brakes is relatively straightforward: Annex 6 to ECE Regulation No. 13 states that "The response time to be taken into consideration for the purpose of the test is that corresponding to an actuating time of 0.2sec. This response time can be obtained from the graph by interpolation.
"For an actuating time of 0.2sec the time elapsing from the initiation of brake-pedal actuation to the moment when the pressure in the brake cylinder reaches 75 per cent of its asymptotic value shall not exceed 0.6sec."
An asymptote is a line which continually approaches a given curve, but does not meet it within a given distance, and the cylinder to be used is "the least favourably placed one". Interpolation in this context means estimating from known values.
The requirement for measuring response time on trailers and semi-trailers is rather more complicated. However, when the jargon is stripped away, it is not especially difficult to understand. A simulator has to be used to replace the drawing vehicle with connections for the trailer's control line and feed line (service and emergency).
In essence the trailer's response time requirement is similar to that for a vehicle, that is the pressure on the "least favourably placed" brake actuator must reach 75 per cent of its "asymptotic value" within 0.4 seconds of the pressure in the control line reaching 0.65 bar (9psi).
The real problems arise when manufacturers try to decide how they are going to measure such small units of time and pressure. Clearly, sending an apprentice to the rear of the trailer to shout when the brakes are applied will not do!
VTAC, the Vehicle Type Approval Centre at MIRA, has built its own equipment, but this is not intended to be either portable or commercially available.
As he became increasingly involved with the new braking regulations, Frank Kirk, of Century Technology, recognised a need for a portable and relatively inexpensive simulator for trailers. Again, the regulations are very detailed about what is required of a simulator and even include two circuit diagrams as examples.
Working closely to these specifications, Frank Kirk designed his machine to be easily portable — it fits into a box much bigger than a commer vehicle battery case — asked Darenth Automati supplier of compressed equipment, to manufacture i1 him.
The regulations demand • each simulator should have calibrating reservoirs and al very little tolerance when r bration takes place.
The specific requiremen1 this:
"The simulator must be set through the choice of orifici accordance with paragraph 3 (which specifies pipe and on sizes) in such a way that, reservoir of 385±5cm3 is joi to it, the time taken for pressure to increase from 0.6 4.9 bar (10 and 75 per cent pectively of the nomi pressure of 6.5 bar) shall 0.2±0.01 second.
"If a reservoir of 1155±15 is substituted for the ab mentioned reservoir the t taken for the pressure to crease from 0.65 to 4.9 bar vs out further adjustment shal 0.38± 0.02 second.
"Between these two pres: values the pressure must crease in an aproxinnal linear way. These reserv shall be connected to the co ing head without using flex pipes and shall have an inte diameter of not less ti 70mm."
Frank Kirk reckons that sr of the biggest problems in bi ing the simulator, apart from obvious one of the very tolerances such as ± 0 second, was the speec switching required and fin( the right kind of integrated cults.
The machine can also be L. for the leak-off test which quires that at a controlled rat pressure drop the trailer brE must begin to operate a pressure of 2 bar or above.
Frank Kirk is now satis with the performance of his production model and Darr Automation has begun to t them. The production mo are even more compact, r suring 400mm x 600mrr 230mm and can be adapter testing powered vehicles as as trailers. They will sell £1,950 each.
• by Tim Blakem ore