Query on "Hydraulic Wedge Brakes"

Rather than hijack someone else's post, though I'd ask in a seperate topic.

Mammoth wrote in his answer on the Bedford Resto page,
"If I am thinking the brakes you describe have the hand brake pull rod pulling on the hydraulic piston. They were around late 40's and fell out of favour by the end of the 50's. Thornycroft used them as well as other pommy makes. Although they do have same action as wedge calling them wedge brakes is likely to cause confusion. The fronts might be plain opposed pistion type as the wedge was about facilitating handbrake over ride on the piston. Auto Surplus might be your best bet for parts".

That sounds about right, the brakes had been stripped down quite a few years ago and luckily had been put in individual cartons.
I have a workshop manual which is for the larger "Gamecock" model of the Karrier Bantam, so at first glance at least look similar.
You may be right, the "WEDGE" component may only refer to the handbrake setup, haven't had a look in the cartons yet.
The terminology threw me a bit, (2 Leading shoes FRONT and REAR) as well as what the book calls "Girling Transverse Wheel Cylinders"
Dave_64

The Girling wedge brakes are in some applications leading shoe in forward and rverse,with floating componenets inside the drum.........the best example is the Army ACCO,which has the full system with pushrods and rockers inside the shoes and floating adjusters and expanders......Most Commers except the #4 eaton axle models have less complex setups ...........both handbrake and service brake work through the same wedge expanders............incidentally,a dangerous situation of seemingly effective brake application through faulty assembly put two bus mechanics in jail and a girl dead some years ago in North Queensland.

My bus has these so called wedge brakes.

The hydraulics or hand brake pulls on a rod attached to a wedge. The wedge forces two mechanical pistons up and down (the pistons are mounted in a tube housing the wedge). The pistons push the shoes (leading and trailing edges) onto the drum.

You have to keep everything clean, greased and adjusted. I reckon a lot of braking force is lost in the mechanics on the wedges.

.
Wedge type braking systems,,( whether they be manual, air or hydraulic ) have accounted for many accidents, (or near misses) ...........
Just because the shoes have been adjusted correctly to the drums , doesnt mean the brakes are going to be effective ..........
At assembly, the travel of the rods (or cylinders)have to be set correctly so that they match the travel of the shoes.........
IE if the shoes are adjusted correctly and there is not much travel in the rods ,cylinders etc it may not be long before the vehicle
doesnt stop too good.........
I have seen brand new vehicles that fell into that category
...................Billy...................

Jon D is on the money. The design eliminates the bell cranks and fiddly bits usually seen in a conventional hydraulic set up to apply the hand brake. Easily identified as the hydraulic cylinder is external with the hand brake pull rod from the diff going into it.
Billy, your big truck wedge brakes are a later generation to what is being discussed, however your comments are just as relevant.

Correct Mammoth.
Here's a picture of the Bedford's bisector (as it's called in the manual.) It only shows 1 piston.

Once, the shoes were a little loose and the bisector pulled to far and popped "out". So the brake wasn't working. I have since learnt to regularly inspect for the actual movement of the shoes when pressing the pedal.
Another problem is the pistons rust up and jam in the "tube.

They are a mongrel to service because you have to remove the axle and drum (bolts) to get to the shoes and guts.

Bedfords version appears different to what I remember For those who are scratching their heads over the diagram the hydraulic cylinder and concentric handbrake pull rod have been cut off at the squiggly bit on the right.
In the ones I have dealt with there is an opposing piston on the bottom, just not seen here as X ray or popping out the bottom.

There are several brands of wedge brakes ...pretty sure Lockheed also made them ,but simpler and cheaper than Girling.,and also Rockwell Stopmasters could be had with hydraulic actuators......In Girlings ,there are early,with sliding mount cylinders,later with tight mount cylinders and bronze expander parts,and the late type with cast iron expander housings......there are also Girlings called Z cams ,which are full air but interchangeable with the hydraulic units...Girling wedges were fitted to cars and motorbikes too.,but a pure mechanical units.