Talk:South African Class 26 4-8-4

Herdner Valves
Contents of Link on Herdner Valves at http://railroad.net/forums/viewtopic.php?f=13&t=70817&sid=2f0e079911a2db9603e6516707f3067b

....most steamers have a Walschaerts, in Germany we only call it Heusinger, gear. The valves open the ports for steam inlet and close these after a certain, by the gear adjustable way, which is also a timing thing.

So 100% means: The complete stroke of the cylinder is open to steam inlet, so 100% of the way, each milimeter of the stroke the full boiler pressure is applying incredible force to the piston.

The question is now: Is this really beneficial? Is this really need?

Let's look on a normal gear: Usually the driver can go from zero up to 65%, some engines have 75% others even 80%.

So 65% up to 80% of the stroke can be done with full boiler pressure to the piston. That means: From the outer end of the stroke over 65% up to 80% of the way the boiler 'hangs' on the piston. For a 1000 mm stroke, as example this means: 650 mm up to 800 mm full boiler pressure can be applied to the piston.

So only the last 200 mm up to 350 mm in the exapmle the inlet is closed and fresh steam supply is cut off. So 350 mm the pressure inside the cylinder drops, as steam expands, but even here, a force due expansion is delivered to the piston surface. So how much do this pressure drop?

Well, assume a 232 psi boiler pressure, so for 65% we can consider 174 psi as release pressure, if the exhaust port opens...

And now: The stroke is not all!

More important is: Which positions do the cranks have?

If the crank is up, we call this 0 degrees position, but this is 50% of the stroke, so piston is in middle position in the cylinder. So zero should be death end position front or rear. So okay, crank at 9 o'clock, wihch means 270 degrees is now set to the zero position.

So 12 o'clock is 90 degrees and other death end is 180 degrees or 3 o'clock, if we assume a forward stroke from rear death end position to front death end position.

Where is the most incredible torque to the axle generated? Of course, if the 90 degrees position, where the largerst lever point and thus strongest force could be applied. For a steam engine, 90 degrees is somewhat incorrect, because this differs a little, because of the drive rod and piston bar angles, but as role of thumb: this 90 degrees angle is okay.

So maximum effort is at 90 degrees, so top up crank position or top down crank position. 12 o'clock or 6 o'clock...

So let's look: At 65% cut off, there is still the full boiler pressure applied to the piston, even at 50% stroke and thus at the 90 degrees angle. So maximum torque effort can be assumed. So the normal valve gear is able to deliver maximum force to the crank, if the driver sets cut off at max.

So when may Herdner valves help?

There is one position, were the cranks often cannont generate enough power to move on the train. This is around the 45 degrees angle. One crank is 45 degrees forward, the other 45 degrees rear... at upper or down half of the wheel, front or rear half of the wheel.. four times each full cycle this position comes... and here the cranks are in realy bad position, because being both beneth the magical point of 75% of the stroke.

So even at full cut off, no steam or only less will enter the cylinders, the engine won't move. Seldom this appears, but it can!

So here a Herdner starting valve overcomes this backdraw on a two cylinder engine, applying extra steam and making the things turn a few degrees, before the main valves admit steam to the pistons...

But usually: Often a little steam enters the cylinder, not enough to pull on, but enough to make the engine move.

So if you wanna go forward, and the engine doesn't move, set back the gears to full cut off backwards (all aft) and look: The engine moves backwards, relieving the couplers of Tender and first coaches or freight cars, and maybe a little more by pushing some way more back. Now the cranks are in a much better position, and even with only 65% max cut off the engine get's now enough steam to pull on at full cut off forward...

A little trick, to overcome this seldom blocking crank position, thus starting valves were found seldom in europe....Allways keep two-thrid level in gauge and a well set fire, that's how the engineer likes a fireman