A balance/friction throttle pull lever for a slide-type carburettor.

[Owen_N]

I am looking at bench dyno controls for my proposed engine.
The carb has about 35mm pull, and quite a strong spring. I need to oppose the spring so that the lever will stay in one place.
The carb spring has a high preload and low rate , so can I do the opposite at the lever end? How would this be arranged?
I had a look a boat throttle levers, but they tend to be expensive- about $122+++ NZD.
*aside*
I also want to hook the lever up to a large servo eventually, so that the loading up of the water brake can be controlled,
I think holding at a fixed rpm, say 7000 rpm, and adjusting brake flow while auto-adjusting up to full throttle at a fixed rpm.
A further enhancement is to have 2 flow valves and a temperature monitor to adjust water flow.
Further again, when the "start run" button is pressed, the engine is accelerated in a steady manner by adjusting the brake automatically.
When it hits the top target overrev, I hit the "Stop" button, and throttle goes to zero, and the brake is shut down.
This will cause a fairly rapid engine stall, but that should be OK.
Keep an emergency ignition cut button in case of engine runaway-an rpm cutout at 19,500 rpm would be a good idea.
*end aside*
I have two carbs, so I could borrow a compression spring from the other one. The spring should run in a tube, as it would be unstable sideways, I think.
The throttle pull cable can run through through the center.
The lever also needs a rotational friction drag, to make it stay put. This could be arranged a bit like a motorbike friction damper, with a friction disc, a backing disc, a finger-type cup spring washer,
and a tightening wheel on the lever pivot.
I was thinking about 6 inches lever length, with a 30 degree throw.

* Question: where could I get one of these fancy cup spring washers?
Would another backing spring layout be feasible?
I could try a search for "rotary friction damper cup spring".
What would be a better search sentence?

 

[Brian Rupnow]

Look for a "Belleville" washer.

 

[Owen_N]

Look for a "Belleville" washer.

I have found them. now I need to work out dimensions and quantity.
I suspect I want about 0.1 N-m, friction discs about 40mm diameter,
Possible axle = m5 screw, dia 5.5mm, or 0.22 "- 7/32" hole diameter. Say, 3x bel. washers, plus a backing washer m5 x 20??
I think this may give me enough friction without squashing the washers flat.
I may need to get another spring the same as the one I have got- the other one is from an 18mm carb, and will be too light.

I will do a search for a PWK 30mm spring. and sheet friction material, maybe 1.5mm thick.
I will sort out a carb cable at the same time - about 1m should be long enough.

 

[Owen_N]

I have found them. now I need to work out dimensions and quantity.
I suspect I want about 0.1 N-m, friction discs about 40mm diameter,
Possible axle = m5 screw, dia 5.5mm, or 0.22 "- 7/32" hole diameter. Say, 3x bel. washers, plus a backing washer m5 x 20??
I think this may give me enough friction without squashing the washers flat.
I may need to get another spring the same as the one I have got- the other one is from an 18mm carb, and will be too light.

I will do a search for a PWK 30mm spring. and sheet friction material, maybe 1.5mm thick.
I will sort out a carb cable at the same time - about 1m should be long enough.

1) It looks like it was just as cheap to buy a whole carburettor, so I ordered a 34mm PWK to go with my 30mm PWK- around $50 NZD
I will see if the engine will run on this. I am not expecting any load until 9,000 rpm, and pipe aim point of 15,000 rpm.
I hope getting spare jets is not so expensive. I will check.

2) For friction material, I got carbonex drag washers, 5mm x 20mm x 0.5 mm.

3) For belleville washers, I will try my cone lock washers in a back-to-back stack, and see how that goes. They are rather thin, so may not apply a good range of loads.

 

[Owen_N]

1) It looks like it was just as cheap to buy a whole carburettor, so I ordered a 34mm PWK to go with my 30mm PWK- around $50 NZD

Here is a layout for my first proposal of partly balancing the spring tension in the carburettor.
This may actually overdo it, and force the lever to the center position.
Setting up the spring in parallel to the cable-pull may be better.
I have put pivots on the ends of the spring, but I am not sure how stable the spring will be with pivots.
The friction discs will overpower the residual spring effect. Switching the ignition off is a good backup.

 

[Owen_N]

Looking at this, I can find tension springs that will work better than this. If I look at a close-to-overcenter arrangement, I can increase pull as the lever is pulled back, which is what is desired. I need to measure preload , travel, and spring rate, to work out what springs I need.
I will draw something up that has an overcenter-type action.

Here is an image of the over-center action.
the original spring is 2.5 N/cm, with 2 cm preload.
The new spring is free length 73.5 mm, 4.6 N/cm, preload at 73.5 stretch to 34 N, pull-type.
-image-
This would be best split into 2 springs , one each side. at 3.4 N/cm. This should be fairly close.
I will recheck before I order the springs.
The initial torque at the lever axle from the carb is 0.33 N-m, and the final torque is 0.89 N-m,
The torque applied by the reaction spring is 34n x 10mm = 0.34 N start;
The final torque = 6.65 x 4.6 = 30.6 N, at 26mm = 0.8N-m
This is fairly close.

 

[Owen_N]

New version rev2, new tension spec. measured at carb:

Springs are ordered - free length 70mm, od 9mm, wire 1mm
-image-
Spring each side, spring supports threaded m5 rod, locknuts
Add clearance for spring mounts.
2 main sideplates for clamping and bottom flanges, truncate one side for rear spring mount and throttle cable clamp.
top throttle handle built up from tubing, washers, bolt and nut.