Press fit a steel pin into aluminum

[Claypoolsbass]

I saw that there was a post on basically the same topic. Strange design, they are asking for .315"/.0006-.0012 on the Dowell pin/8mm then add Loctite 609. I've been in this industry for about 30 years and have never seen that. Any ideas?

 

[SwinnyGG]

Near-interference fit with retaining compound? Not rare at all. Might be done for any number of reasons.

 

[hydtools]

The technique permits use of steel pin without potential galling of the aluminum hole.

Ted

 

[John Donlan]

I think the use of loctite or any other adhesion substance in securing a steel dowel pin in aluminum is totally conditional upon the environment under which this joinery will be exposed. Personally, I think an interference press fit is always better than adhesion with glue in a slip fit situation.

If the pin and aluminum will not be exposed to Great increases or decreases in temperature, then the coefficient of expansion of each of the two metals may not be of great significance and in that case use of an adhesive with a slip fit may work well. However, if this joinery will be submitted to environments of great swings in temperature - either extremely cold or extremely warm - the two materials, steel and aluminum, expanding and contracting different amounts and at different rates..... The joinery may open up and possibly become loose.

Also, you have to consider the conditions under which loctite or any other adhesive will be exposed.... And basically ask the question "will it hold up?" Let me offer a practical example that happened at an aerospace company I was working at.

We were manufacturing linear and rotary actuators for Boeing 737 aircraft. The gear housings were T-356 sandcast aluminum. There were three bearing bores that had to be cut into the internal bottom of the casting. The diameter was something like 1.375 +.0000/-.0005". This was designed to be a press fit situation with an interference between .0005 - .0010". This type of tight tolerance on a bearing bore on sandcast aluminum was difficult to maintain on a constant run of parts. This type of aluminum would have sand inclusions that would greatly wear away the cutting surfaces of bearing bore reamers quickly. So one day some people in the design engineering department decided to increase the diameter of the bearing bores so as to allow a very close slip fit (.0005 - .0015 diametrical clearance). The bearings were to be sealed into the bearing bores using loctite.

Unfortunately, the design engineers did not take into consideration the environment under which this loctite would be exposed. When a Boeing 737 aircraft is at cruising height the ambient temperature around these linear and rotary actuators falls to something like minus 40° below zero. What would happen is.... The loctite would freeze and crystallize and become loose particulate within the gear housing assembly, almost like a vapor within the entire gear housing. When the plane descended down to ground level temperatures the loctite "fog" would change from crystalline form back into a more solid state. Of course by this time these loctite crystals had coated all of the shafts and gears within the gear housing, causing the gears and shafts and bearings to all seize! Now, when you're depending upon these actuators to control the vertical and horizontal stabilizers, flaps, elevators, etc. Of the jet aircraft..... Having them all seize up is not a good thing. DUH?!??

Thank goodness the company was able to trace down all of the linear and rotary actuators employing this loctite situation before they could be installed on the aircraft.

So you can see how important it is to understand the environment under which the steel pin and aluminum have to be exposed. Temperature, pressure, humidity.... These all can effect how well loctite may work or not in the life cycle of the joint between the pin and the aluminum.

At the end of the day, probably an interference press fit is the best solution.... And the joint that will hold up best under most conditions.