Post by suctionhose on Jul 3, 2024 12:00:45 GMT
You don't see much on suspension so I thought I'd put this in General as well as on the Build Threads for wider appeal. The following relates to the 5"g NZR 4-6-2 I'm making.
The locomotive frame, without tender, is over 1300mm long (4ft 4ins) which is pretty long for 5”g. I want it to go round 9m curves (30ft Rad) and keep the cowcatcher clear of the track on a 9m Rad vertical curve like coming off the ground level up to a raised steaming bay.
Here’s the challenges:
1. Sufficient side displacement of front bogie (and rear truck when we get to it)
2. Side control / centring device capable of large side displacement
3. Clearance of bogie wheel & cylinders on tight curves
4. Near optimum adhesion without equalised suspension
5. Sufficient vertical travel among axles to keep drivers firmly planted
6. Stability from rocking or pitching when pulling hard
A study of Equalised Suspension systems used by the Americans, reveals ‘grouping of wheels’ in specified ways according to wheel arrangements providing a central point of support in the first group and points of support either side in the second group, making a three-point triangle upon which the sprung proportion of the locomotive sits. Weight is distributed across multiple axles by the equalising beams, which are usually not symmetrical, to equalise axle loads.
Typically, a 4-6-2 has the front bogie as the first group and the 3 driving axles AND trailing truck as the second group. Centre of Mass, excluding wheels, axles, motion (all unsprung weight) is split between the two groups – think of a beam or see-saw with the fulcrum placed to distribute the loads in the required proportion – more on the drivers than the bogie.
Ok. So that being very interesting, I have no wish to make or maintain hundreds of parts for equalised suspension! I need a simplified principle to get the best out of individually, coil sprung axles. And a bogie that will be the equivalent of a “4WD off road version”!
I’ve settled on the principle that my triangle of support will be the centre of the front bogie and the two Trailing Coupled Wheels for reasons that are not obvious until you draw it. The bogie will be unsprung except for where the load is applied at the centre pivot / bolster.
These three points will have very stiff springs and very limited movement to prevent the loco from pitching and rocking. The Main and Leading Coupled Wheels will have long, soft springs – compressed to equal axle loadings – and plenty of vertical travel to meet undulations in track without significant change in axle load.
This sketch shows the effect of humps and hollows at the extremes of a 9m vertical radius. You can see the trailing coupled axle at a fixed height, the front bogie pitched up or down about the swivelling bolster to conform to the track surface. The leading coupled and driving axles rise over the hump or drop down in the hollow courtesy of long travel springs and allowed movement in the horns.
Springs:
Spring Rate refers to the ratio of force vs deflection. A short spring wound from heavy wire has a high Rate meaning a large change of force produces a small deflection. A long spring wound from lighter wire has a low rate and will deflect a lot more for the same change of force.
The short Brown Spring will be used to ‘fix’ the trailing axle within very small limits of movement. The long Yellow springs will be used on the leading and driving axles to provide long travel with very slight changes in load to suit the humps and hollows described above.
Both springs will carry the same axle load however, Yellow will be compressed considerably further than the Brown. Springs are located above the axle boxes in holes drilled up into the16mm thick frame. Adjustment is achieved using spacers above in the blind holes.
Front Bogie:
Reference to the sketch above shows the bogie pitching up or down as required. This is accomplished by a swivelling bolster in the truck which also allows a great deal of freedom for the truck to twist without bottoming out springs or being victim of excessive spring rates. The aim is to balance the loco with 10kgs on the bolster (with a heavy spring for shock absorption) thus ensuring the front of the loco lifts when required to keep the cowcatcher clear of the track.
To prevent the issue of front wheels hitting the cylinder on tight curves, the bogie wheel centres are 10mm longer than scale with the leading wheels pushed out the extra 10mm ahead of the cylinders. It's unnoticeable. Side control or self-centring is yet to materialize and will be further revealed when photo’s are available. Suffice at this point to say that 9m rad curves are very tight for fixed frame of such length so a concept somewhere between Fullsize and Hornby 0 Gauge is required.
Weight Distribution:
Summing the estimated weights of frame, cylinders, boiler and accessories – (excluding the unsprung components i.e. wheels, axles, motion) – I’m guessing 75kgs. I want the distribution front to back to be 10, 20, 20, 20, 5kgs. From the diagram below one can sum the Moments about FB (front bogie centre point) for each axle and resolve them into a single 75kg x 429mm Moment. That is, the CoG needs to be 29mm ahead of the driving axle. Application of counterweights if needed.
The locomotive frame, without tender, is over 1300mm long (4ft 4ins) which is pretty long for 5”g. I want it to go round 9m curves (30ft Rad) and keep the cowcatcher clear of the track on a 9m Rad vertical curve like coming off the ground level up to a raised steaming bay.
Here’s the challenges:
1. Sufficient side displacement of front bogie (and rear truck when we get to it)
2. Side control / centring device capable of large side displacement
3. Clearance of bogie wheel & cylinders on tight curves
4. Near optimum adhesion without equalised suspension
5. Sufficient vertical travel among axles to keep drivers firmly planted
6. Stability from rocking or pitching when pulling hard
A study of Equalised Suspension systems used by the Americans, reveals ‘grouping of wheels’ in specified ways according to wheel arrangements providing a central point of support in the first group and points of support either side in the second group, making a three-point triangle upon which the sprung proportion of the locomotive sits. Weight is distributed across multiple axles by the equalising beams, which are usually not symmetrical, to equalise axle loads.
Typically, a 4-6-2 has the front bogie as the first group and the 3 driving axles AND trailing truck as the second group. Centre of Mass, excluding wheels, axles, motion (all unsprung weight) is split between the two groups – think of a beam or see-saw with the fulcrum placed to distribute the loads in the required proportion – more on the drivers than the bogie.
Ok. So that being very interesting, I have no wish to make or maintain hundreds of parts for equalised suspension! I need a simplified principle to get the best out of individually, coil sprung axles. And a bogie that will be the equivalent of a “4WD off road version”!
I’ve settled on the principle that my triangle of support will be the centre of the front bogie and the two Trailing Coupled Wheels for reasons that are not obvious until you draw it. The bogie will be unsprung except for where the load is applied at the centre pivot / bolster.
These three points will have very stiff springs and very limited movement to prevent the loco from pitching and rocking. The Main and Leading Coupled Wheels will have long, soft springs – compressed to equal axle loadings – and plenty of vertical travel to meet undulations in track without significant change in axle load.
This sketch shows the effect of humps and hollows at the extremes of a 9m vertical radius. You can see the trailing coupled axle at a fixed height, the front bogie pitched up or down about the swivelling bolster to conform to the track surface. The leading coupled and driving axles rise over the hump or drop down in the hollow courtesy of long travel springs and allowed movement in the horns.
Springs:
Spring Rate refers to the ratio of force vs deflection. A short spring wound from heavy wire has a high Rate meaning a large change of force produces a small deflection. A long spring wound from lighter wire has a low rate and will deflect a lot more for the same change of force.
The short Brown Spring will be used to ‘fix’ the trailing axle within very small limits of movement. The long Yellow springs will be used on the leading and driving axles to provide long travel with very slight changes in load to suit the humps and hollows described above.
Both springs will carry the same axle load however, Yellow will be compressed considerably further than the Brown. Springs are located above the axle boxes in holes drilled up into the16mm thick frame. Adjustment is achieved using spacers above in the blind holes.
Front Bogie:
Reference to the sketch above shows the bogie pitching up or down as required. This is accomplished by a swivelling bolster in the truck which also allows a great deal of freedom for the truck to twist without bottoming out springs or being victim of excessive spring rates. The aim is to balance the loco with 10kgs on the bolster (with a heavy spring for shock absorption) thus ensuring the front of the loco lifts when required to keep the cowcatcher clear of the track.
To prevent the issue of front wheels hitting the cylinder on tight curves, the bogie wheel centres are 10mm longer than scale with the leading wheels pushed out the extra 10mm ahead of the cylinders. It's unnoticeable. Side control or self-centring is yet to materialize and will be further revealed when photo’s are available. Suffice at this point to say that 9m rad curves are very tight for fixed frame of such length so a concept somewhere between Fullsize and Hornby 0 Gauge is required.
Weight Distribution:
Summing the estimated weights of frame, cylinders, boiler and accessories – (excluding the unsprung components i.e. wheels, axles, motion) – I’m guessing 75kgs. I want the distribution front to back to be 10, 20, 20, 20, 5kgs. From the diagram below one can sum the Moments about FB (front bogie centre point) for each axle and resolve them into a single 75kg x 429mm Moment. That is, the CoG needs to be 29mm ahead of the driving axle. Application of counterweights if needed.
