SP MK10 PLUS 1ST GENERATION.....?

[axxel57]

Hey guys!

Has ever anybody of you seen this model of MK10 Plus?

For me it looks as it must be the very first generation of the European MK10 Plus from 1995 ( same year when the brass tip piston for the MK20 was implemented I think), or a very first Upgrade Kit has been used.....

Anyway, it's another first for me, and for you?

 

[Mobulai]

Hey guys!

Has ever anybody of you seen this model of MK10 Plus?

For me it looks as it must be the very first generation of the European MK10 Plus from 1995 ( same year when the brass tip piston for the MK20 was implemented I think), or a very first Upgrade Kit has been used.....

Anyway, it's another first for me, and for you?

I met my first brass tipped piston (on an mk20) this week — your mk10+ seems like a singleton

Speaking of, the piston bullet didn’t seem to fit it, is that the case for this mk10+ as well?

 

[axxel57]

No, the common piston bullets don't fit, but with these types of pistons you don't need them really, the piston orifice is very blunt........

 

[Mobulai]

No, the common piston bullets don't fit, but with these types of pistons you don't need them really, the piston orifice is very blunt........

Do they also tend to IP „drift“ because of that extra bluntness?
After service and micromeshing IP locks at 120, and then within 5 minutes it’s at 135, then stable overnight

 

[axxel57]

Yes, for sure, this is why they were only very short on the market, the brass is symply too soft, the slightest dirt on the orifice makes them notorious creepers.
The design was supposed to improve the balancing properties of the pistons, but they became those notorious creepers and diappeared after only one year or so.

If you buy old MK20 model, those pistons are the only ones you have to change according to SP.

 

[halocline]

The design was supposed to improve the balancing properties of the pistons, but they became those notorious creepers and diappeared after only one year or so.

Do you mean the brass tip was supposed to reduce the amount of IP rise at high supply pressure? If so, how? From what I know, that rise is due to any amount of horizontal surface area that allows for the HP air to ‘push’ on the piston edge, and of course some of the rise is due to increased pressure gradient on the HP piston o-ring which causes an increase in friction on the piston shaft.

But unless the geometry of the brass piston tip is different than the steel rounded piston edge, I don’t see how it could have that effect.

I’ve never seen a brass tipped MK10+, only a couple of MK20s which I immediately replaced with the composite piston.

 

[axxel57]

As I understand it, the balancing properties of a Flow Through Piston depend on the ratio of the piston stem diameter at the HP o-ring and the piston sealing edge diameter.

On the straight steel piston stems there must be always a slight difference between these two diameters.

If the piston stem diameter is just a little bigger than the piston sealing edge diameter, the IP will decrease with lower tank pressure.

If the piston sealing edge diameter is just a little bit bigger than the piston stem diameter at the o-ring, the IP will increase with lower tank pressure.

If you manage to produce a piston with an equal diameter at the piston stem at the o-ring and the sealing edge, the IP should be equal at full and near empty tank, at least in theory.

Since the brass tip pistons are clearly a bit wider at the top of the piston than at the stem middle part, I assumed that SP was trying exact that with the brass tip.

I didn’t have any other explanation why SP should use such a soft material at this crucial point.

If you look at the Composite Piston, the top of the piston is not straight as the first generation pistons, but also slightly wider than the piston stem in the middle, the sealing edge seems to be ‘bend’ slightly outward, but the steel is much harder than brass, so one could design the sealing edge slightly wider.

To me it seems as if the sealing edge could be slightly wider than the piston stem, this would explain why in contrast to the old straight steel piston design the IP is increasing with the lower tank pressure.

I might be wrong with this, because it is for me very difficult to measure these very small differences in diameters (my caliper is simply not accurate enough), but that is how I understood up to now this issue…..

 

[Tanks A Lot]

The concepts above are exactly right. I had a discussion with someone else, where I laid out how I usually explain the concept. Maybe it is of some interest to others. I believe it is so hard to understand, because we have a rather poor intuitive feeling about pressure. Furthermore, the concept of how a diaphragm first stage is balanced is usually taught very poorly, putting great emphasis that there is pressure on both sides of the poppet, which is a happenstance of the true reason for balancing.

---

Part 1:

---

Imagine a gas as millions of tiny sand particles moving randomly within a given space. These grains of sand collide with each other and with the walls of the enclosed space in an isotropic (random) manner. The more grains present, the more frequent the collisions; the fewer grains present, the fewer the collisions. Whenever a grain hits the walls, it imparts a tiny amount of force onto the wall. The more grains of sand that hit the walls, the more force is transmitted. This is exactly what we call pressure—the collisions of molecules with the walls of the container. More collisions mean more pressure; fewer collisions mean less pressure.

Pressure is defined as a certain force acting on a given area. We can express this relationship with the following formula: Force_F = Pressure_P x Area_A

Now, here’s the crucial point to understanding pressure: it always acts perpendicular to a surface. We’ll explore why this is below, but remember: it never acts sideways on a surface, never! The grains of sand hit the surface from random angles, in fact, from every conceivable angle. Some hit the surface at a very shallow angle from the left, some from the right, and others directly from above. Those hitting at shallow angles impart a small amount of perpendicular force and a large sideways force. However, the sideways forces always cancel each other out on average. For every particle hitting the wall at a shallow angle from the right, there is another particle hitting it at the same shallow angle from the left!

The sideways forces imparted by the blue, teal, and purple particles on the surface cancel each other out because they are equal and opposite. However, the tiny (depending on the angle) perpendicular force they impart is not opposed by any other force. The only forces that remain uncancelled are the collective perpendicular forces acting on the surface area, which is why pressure always applies perpendicularly, never sideways.

Let’s examine how this perpendicular force acts on a very simplistic piston. We intuitively understand what happens to the piston in the following scenario:

The 10bar pressure in the left chamber pushes the piston toward the right chamber, where there is only 1 bar of pressure. What may not be immediately obvious is that only the pressure acting directly on the piston head on the left side is responsible for this movement. This is the purple force in the illustration. The blue forces cancel each other out exactly and play no role in the piston’s movement. For now, we’ll ignore the small force from the 1 bar of pressure acting on the right side of the piston head, as it’s not relevant to our discussion.

 

[Tanks A Lot]

Part 2:

---

Now, let’s modify our drawing slightly. Suppose we’ve welded the piston to the left side of the container. There is no air at all between the piston head and the left wall; they are mechanically joined.

We can redraw the perpendicular forces, and unsurprisingly, there are no net forces acting on the piston. The perpendicular forces (drawn in blue) cancel each other out exactly, just as they did before. However, the difference this time is that there are no forces acting on the piston head, as no air is present between the wall and the piston head to exert any pressure.

You may notice a slight change in the forces acting around the O-ring. The forces acting on the O-ring would certainly push it outward. These forces were present in the previous illustration as well, but I omitted them for clarity. But remember, it is the O-ring getting pushed, not the piston. There is not force that can push the piston towards the right, as they all act perpendicular to it.

Now, let’s revisit the balanced piston, but this time with a piston that is free to move and not fixed to the walls of the container.

You’ll notice there is virtually no difference from the previous illustration. How could there be? The high pressure is still unable to pass between the piston head (now forming the orifice) and the wall of the container (now occupied by the seat). The perpendicular forces act on the walls of the piston, cancelling each other out. From this, it’s clear that no matter the supply pressure, whether 10bar or 100bar, as long as the piston is closed and touching the seat, there are no net forces acting on the piston from the high-pressure side. All forces are perpendicular to the piston, and none of these perpendicular forces point in a direction the piston could move.

The above is an idealistic view of how the piston is balanced. In reality, the edge of the piston (the orifice) cannot be machined perfectly flat and sealed; this is mechanically impossible. There must be an edge machined into it, and no matter how sharp I make it, this edge will always be slightly thinner than the rest of the piston and, more importantly, the inner sealing diameter of the piston O-ring. The illustration below is greatly exaggerated, but the principle holds true in real-world scenarios.

Notice how the purple forces are still perpendicular to the edge (orifice) of the piston. However, they are no longer perpendicular to the shaft of the piston. As a result, these forces impart a tiny force pushing the piston toward the lower-pressure side. This tiny vector of forces pushes the piston to the right, which is why this design is not truly fully balanced. Supply pressure does play a role in opening and closing this mechanism and will change with changes in supply pressure.

Engineers have become increasingly clever and worked around this limitation as well. By increasing the orifice of the piston, we give the high pressure a larger area to act against, pushing the piston toward the closed position. This means I need a lower intermediate pressure to close the piston, as part of the work is done by the high pressure.

The opposite is also possible: by effectively making the orifice smaller than the piston shaft, I can use the high pressure to assist in opening the piston.

However, in reality, the engineer doesn’t want the high-pressure supply to play any role in opening or closing the piston. Therefore, they slightly widen the orifice or, more precisely, thin the diameter of the stem. They ensure that the orifice diameter after having a edge machined into it matches the stem diameter, or more accurately, the inner sealing diameter of the stem O-ring exactly.

---

The one thing I usually omit is how the O-ring on the piston shaft interacts with things. In the above I have dismissed it, which is not entirely correct. There is an ever so slight downward force onto the piston from the O-ring dragging on it. In an ideal world the friction would be zero, but even with the best lubricant there is still some tiny friction left.

In the end I find this fascinating from an engineering point of view. From a practical standout I find it all but irrelevant. Those tiny drops of not being fully balanced don't make any difference in the real world, it is a gimmick, nothing else.

 

[Pressurehead]

I was keeping up with this until part 2.
Then my eyes glazed over and needed a lie-down to try to digest part 1 before reading part 2.

I am overwhelmed, the Knowledge base here is impressive.
Please don't stop, I am loving it, just 'thick as a brick' slow on the uptake.