Wednesday, April 18, 2007

SON Dyno Hub Pressure Compensation Device

I found the following information describing the functioning of the SON pressure compensation device quite interesting:

Bicycle Quarterly article on generator hubs at

Comments by Peter White on the Randon List:

"Jan's article is excellent. But there are a couple of points I'd like to make. He refers to the "pressure compensation seals" on the newer SON hubs. But calling it a seal might be a bit misleading, and Schmidt actually calls it a "pressure compensation device". The seals in the old
hub and the new hub are identical, being built into the cartridge bearings themselves. The "PCD" just makes those seals more effective.

It's a system which allows water to enter the hub, but only in a tightly controlled way. With the original design, when the hub cools, air gets sucked through the cartridge bearing seals into the hub shell, which has a lot of air inside. So when its pressure changes, a lot of air wants to move either into or out of the hub. If the air is cooling inside, and therefor contracting, it creates a suction inside and pulls air in through the "sealed" cartridge bearings. If there's water present at the outside of the bearing seals, it will get sucked in along with the air, and of course damp air will condense on the cold metal surfaces inside the hub. This happens with all hubs, but the effect is greater in a dynohub because of the increased quantity of air, and the critical gaps between stator arms and magnets.

Once that water gets inside, it can and will cause corrosion on the steel dynamo, narrowing the tight gap between the stator arms and the magnets, and causing the dynamo to lock up.

The new version has a little hole in the axle, about a third of the way from the contacts end, and a thin plastic tube attached which is then wound around the stator inside. I don't recall how long the tube is, probably about a foot or so. When the air pressure inside the hub shell drops, air is sucked into that plastic tube, and if there's any water inside the axle, it will get sucked in as well. It's much easier for air and water to move through the axle and that plastic tube than it is to move past the bearing seals, so no water or air moves through the seals in the newer hub, it only moves through the plastic tube. And the tube is long enough, taking up almost all of the available air space inside the hub shell, that water never gets beyond the end of the tube thanks to capillary action, and never gets to the steel parts inside. Since water never gets to the steel parts, they never rust, and the hub won't lock up.

So, the pressure compensation device effectively makes the bearing seals far more effective, by making an easier path for air and water to move through. And that's why it's so important that the user not heavily grease the skewer in a SON hub. If grease clogs the hole in the axle for the PCD, water will then get sucked in through the bearings, and, well, you know the rest."

1 comment:

bmike said...

I wonder how much grease is "not heavily"...
I'll have to go check mine before my ride. I used to change skewers depending on which bike I was riding... EEK!