Locktite allowance

I have just had a braked own due to a failed Locktite jointed sleeve.

I think it’s due to too much clearance. I used 601 which needs a gap of up to 4 thou.

The big question is does this apply to the total gap between the parts. ie bore dia minus 4 thou per side giving 8 thou difference total, or is it just 4 thou difference.

The part was right on the limit of this size at 4 thou gap all the way round it should have been smaller, and the surfaces were fine machined which probably didn’t provide enough key. When the sleeve was pulled out the Locktite came out as a paper thin film 4 thou thick. Perhaps not clean enough as well!

4 thou should be OK IIRC, but thinner is probably better. Cleaning is important but Loctite 603 (hi-strength) is more tolerant. of poor cleaning according to the blurb.

When using Loctite for retaining parts I have always tried to machine the parts to have a very light slip fit and so keep the clearance to a minimum without it having any interference. The adhesive needs to have some thickness to work effectively. The surface finish does not need to be superfine so as to provide a key in both parts.

Depending on the particular grade that you are using cleaning is important. I always use one of the brake and clutch cleaning sprays to remove any oil from the parts regardless of what the TDS says, just to be sure. Any cleaning fluid must be allowed to evaporate completely otherwise the Loctite will not cure properly, usually ends up as a sticky mess if you don't.

I tend to favour 648 for my permanent fits, mainly because this is what I have. I notice that 638 has a higher strength rating, but cures more slowly. Both have a high temperature rating. It will depend on what is available to you and what your particular application requires.

Neil

In 2014 I posted

"My reading of the guidance can be summed up in a few rules of thumb:

• Loctite's primary purpose is for bonding round shafts in circular holes.
• If the joint has some mechanical strength already (i.e. press fit) it will be stronger.
• Loctite will gap fill up to 0.3mm, but the joint gets stronger down to 0.005mm
• For optimal strength he joint gap should be no more than the thickness of a sheet of paper but it cannot be too small.
• For accurate alignment, an interference fit is best
• The joint surfaces should be abraded with emery to maximise adhesion and ensure adhesive is carried into the joint

This would suggest the 'cotton reel' is unnecessary, but if it reduces the force require or permits rotational adjustment before setting a shallow 'cotton reel' to allow hand assembly while maintaining alignment is a good idea."

The thread with links to a 'white paper' on best practice is here:

www.model-engineer.co.uk/forums/postings.asp?th=101084&p=1

Neil

The only failure I've ever had with loctite turned out to be from using (very) old stock. The techy people at loctite recommended getting the old stuff off with paint stripper before cleaning and re-applying. If it's done properly it is very strong, I once blew up a 1te hydraulic jack trying to get a 5"g trolley wherl off its axle. We always think that a bit of pipe over the handle will give us just enough extra

The usual clearance should be of the order of 0.002 - 0.003" (0.050-0.076 mm )

The parts should be cleasn, and free of oil or grease.

I used Loctite between the faces of a collet chuck and a fabricated backplate, to hold them oncve the nbtaper hadv been clocked trur and nthe bolts tightened.

When I wanted to separate them, to fit the chuck to a machined cast backplate., they could not be separted.

Really heavy blows with amallet would not shift them, so theyb remain together.

Howard

0.1mm (on diameter) is the max it will work with but you could go a lot less.

How long did you let it cure before it failed? the larger the gap the longer it will take to reach full strength, same applies to materials being bonded as they also affect cure time.

601 TDS 

Diametrical Clearance

Edited By JasonB on 22/08/2023 07:26:32

Ok thanks folks. Even the data sheets don’t indicate if it’s radial or diametric clearance. It is constantly referred to as gap which suggests radial, although it gives info about press or shrink fit which in reality means zero gap!

so the new sleeve will be made with a small hand slip fit clearance

Jason. 10 years!

Howard if you want to separate put it in the domestic oven!

What metals are involved, as some don't work well and may need the use of an activator/primer, to improve the final joint strength.

Edited By Nick Hughes on 22/08/2023 10:30:49

Zan, see my second link where it clearly stated Diametrical clearance twice, the second time is >0.1mm  <0.1mm

Edited By JasonB on 22/08/2023 12:21:00

… and, at the risk of stating the obvious : The key features of Loctite 601 are

• Medium cure speed
• 1-part product, requires no mixing
• Low viscosity, small gap filling
• High strength for permanent bonds
• Fluoresces for presence monitoring

That low viscosity is designed-in, to let it do one type of work rather well.

Other numbers, like 638, have much higher viscosity and will centre a loose-fitting shaft quite nicely with a ‘quick twirl’

.

MichaelG.

Psst … <0.1

MichaelG.

Have 601 and 638, which will be best for 6mm diam pin in a 3mm deep blind hole CZ121 brass/brass joint ?

Emgee

I would use 638 … but do beware that you may need a little groove [or a rather loose fit] to let any excess escape from a blind hole, and most importantly that it will probably harden quicker than you expect. … Copper is an accelerator for the reaction.

MichaelG.

Edited By Michael Gilligan on 22/08/2023 14:46:31

read the spec ,do what it says ,work's every time. must remember to do this myself. used a Loktite recently [ not at home so can't check which one] spec says sets in 1/2 hour but full strength one week

I’m going blind n it seems I can’t read data info!….. thanks again

Hi Martin

Loctite spec I found only mentions gap and time to cure, no detail of materials.

Emgee

Edited By Emgee on 22/08/2023 22:38:18

Emgee, the TDS sheets have a graph showing cure time and max strength for common materials, first grah on the 601 TDS I posted yesterday.

Yes thanks Jason, I saw those and similar on-line but they refer to steel components, could't find info for brass.

Emgee