Telma for trucks -the pros and cons
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By Gibb Grace,
DAuE, CEng. MIMechE MOST COACH OPERATORS are familiar with Telma electric retarders bin it is probably true to say that most truck operators are not. Even less appreciated is the fact that Telma produces a range of retarders, each one designed to suit a specific vehicle weight. By the very nature of things a coach weighing, say, 10 tons, does not need as big a retarder as a 32-ton truck.
Not only the model of retarder is different but also the installation in the vehicle. Broadly speaking two types of installation are possible; for long-wheelbase, lighter vehicles the retarder is best placed at a break in the propshaft, say at the centre bearing. But for heavier vehicles, and particularly for tractive units, a relatively new installation adjacent to the driving axle has been developed. This type of installation is called FOAL. In this country some 300 vehicles have so far been fitted with the FOCAL type of retarder and a further 1200 or so on the Continent: In spite of the Telma being an effective auxiliary retarder many operators in this country remain sceptical. This is mainly because it is an electromagnetic system and inherently heavy. It is significant, I think, that in France and Switzerland a vehicle's normal maximum gvw is officially allowed to be exceeded by the weight of the Telma installation, thus effectively removing the payload deficit which it inevitably incurs.
The largest unit available, and therefore the one giving the best braking effect, is excessively heavy for British trucks and the best suited unit for vehicles between 20 and 32 tons gvw is the FOCAL 155 which with its mounting weighs approximately 4cwt. The FOCAL type does not have its own bearings and relies on the pinion bearings in the axle differential head ta support its extra weight. Not all Of the 4cwt however, is carried by these bearings, only the weight of rotor which in the case of the FOCAL 155, is 1.25cwt. The stator weight is supported by the mounting bracket which is attached directly to the axle casing itself.
One day to install Installation of the unit takes one day at Telma's London factory and involves shortening the propshaft by a few inches. If axle pinion and gearbox output shaft alignment under driving torque is likely to be affected by the shorter propshaft, the inter-axle and spring packing wedges are altered to prevent this happening.
Because the retarder is electromagnetic in nature it does use considerable electrical power — up to 3kW — and so generates a lot of heat. If the retarder is used continously for a long time, say 15 to 20 minutes, temperatures up to 400deg C can be reached in the rotor. I asked Telma why temperatures as high as this did not brei down the axle lubricating oil and simf demonstration provided the answer. watched •a small rotor being balanced, Telma's London works, and while spinnii at a typical speed the air flow through could be plainly seen and felt. The ca steel rotor has fan-like blades which dra cool air from the central bearing area al centrifuge it outward, past the hot rim the rotor into the air stream.
The heat produced, then, does n affect the axle lubrication, but it does affe the ultimate braking performance of ti device. Performance drops by as much 50 per cent between cold and hot operatic but I was assured that a steady sta performance is reached beyond which the is no further deterioration in performance. is, I think, fair to say that very few, if an trunk routes in the UK would take tl Telma to this performance limit, but Sth
utes are common in the Alps and the rrenees and it was for conditions such as ese that the Telma was developed to ovide continuous braking.
ontinental concern Continental vehicle Manufacturers and wernments generally seem to be much ore concerned with the question of ixiliary braking and it is an indication of .e acceptance that the retarder has thieved in that future trucks are often anned in co-operation with Telma.
To get the views of a user and to try the elma for myself I spent a day with John antrell, an owner-driver operating in and at of Buxton, Derbyshire. Mr Cantrell's --year-old Seddon hauls a tipping :mi-trailer and runs at 30 tons gcw in me of the toughest country in Britain. efore the Telma was fitted the tractive unit rakes needed relining every 20,000 miles ad the semi-trailer brakes every 15,000 tiles, not to mention new brake drums very year. Last September with 188,000 iiles on the clock the Seddon brakes were ompletely overhauled and a Telma OCAL 155 retarder fitted and when I met Ir Cantrell in May 1972 the unit had overed a further 35,000 miles of omparable operation with only minimal fling wear. Mr Cantrell does all his own maintenance and knows the vehicle backwards and judging by the few brake adjustments he has made to date he reckons this current set of linings will last him about four years.
Four levels of braking I drove the outfit for some 20 miles which though not long enough to use the retarder to best advantage, was sufficient to convince me of the safety margin it provides. The retarder is silent and smooth in action, it is very simple to use and has four levels of braking available. At first one tends to apply too much brake too quickly, such is the braking force available, and thus make ragged progress but in time one becomes more accustomed to using it and one's performance begins to improve noticeably. John Cantrell reckoned three days or about nine times over his route was all that he needed to master it. Certainly he handled his outfit beautifully on the outward journey, descending the four-mile-long Goyt Pass without once touching the service brake. The combination of 5th gear and three notches on the Telma held the
vehicle to a fast but safe .35 mph on this 1 in 12 twisting descent. The retarder was not only used on downhill sections but also in place of the service brake in normal road situations thus further saving lining wear.
A FOCAL 155 unit costs £299 and fitting a further £100 which represents a considerable extra outlay on the average heavy truck but one must weigh against this the added safety margin and the reduced brake maintenance costs. Depending on the type of operation involved the initial outlay could be recouped within two to five years.
Telma users, as 1 discovered, are always looking for more retarder performance; some coach operators, I was told, would like to do away with conventional brakes altogether, so heavy vehicle operators should be pleased to know that a new model the FOCAL 170, is now available. It is no bigger than the 155 model but because of further electrical refinements produces a greater braking effort. The price of this unit is £339 plus £100 for fitting from Telma Retarder Ltd, 841 Yeovil Road, Trading Estate, Slough, Bucks.