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Tech Info - Adhesives

The two main types of adhesive used in the vehicle builds shown on the site are conventional PVA type wood glues and the slightly less well known but still readily available industrial type super glues or cyanoacrylates.

Conventional Wood AdhesiveWood adhesive probably doesn't need much of an introduction. I use a relatively quick setting weatherproof formulation of the type available in most hardware/DIY stores and find I generally obtain high strength trouble free joints in the timber chassis structures. The usual tips apply - make sure the surfaces to be bonded are dry and clean, if possible load the joint while the adhesive sets and, whilst the glue is wet, clean off the excess squeezed out of the joint with a finger or cloth. - simple (health & safety note - don't eat wood adhesive!).

One point that is worth making is the importance of straight cut flat edges on the parts to be joined. To achieve strong joints the contacting faces of the joint must be brought into intimate contact with each other. Unevenly cut edges on the parts to be joined will produce high points and, in consequence, will also produce gaps in the contact areas between these high points - this loss of bond area does not make for strong adhesive joints. Machine cut edges or edges planed or sanded dead-straight after cutting are best - it's worth taking the time to get this right before reaching for the adhesive bottle. 

 

 

Clamped joints during settingA second important point to consider is that although wood glue is tremendously strong when set, the adhesive joint's strength is off course also limited by the strength of the parent materials. This is particularly important when a structure requires bonds made to the face of plywood sheet - the strength of the adhesive used in the manufacture of the sheet to bond the individual plies may not match that of your wood glue and such joints placed in tension may fail through delamination of the sheet. In these circumstances consider reinforcement of the bonded joint with screws or other mechanical fasteners.

Instant Adhesives - I've used cyanoacrylate instant adhesives for some time now for fixing steel chain sprockets, belt pulleys and gears onto steel drive shafts. This is as an alternative to conventional power transmission component fixing methods such as keys or splines which I've tried to avoid in my pursuit of simple manufacturing technology designs. Using instant adhesives to do this job has proved remarkably effective in the modestly powered EV's I've built, although there will be a shaft torque level at which bonding alone will not be sufficient.

Simple straight bored holes in the sprocket/gears and unstepped parallel shafts can be used - no steps or grooves need be machined in the shaft, although the joint diametral clearance does have to be controlled. Standard sized bright steel shafting can be used with the sprocket/gear bore drilled to give the required fit. To bond steel to steel I use Loctite 480, this is a rubber toughened cyanoacrylate with enhanced shock resistance and, very usefully, a slower cure speed than most instant adhesives. On steel/steel joints it sets in  60 to 120 seconds which gives time to work the adhesive fully into the shaft/hole gap to produce as closely as possible a full area bond. Full strength is developed after curing for about 24 hours at room temperature. Shear strengths for Loctite 480 on steel are quoted at 22 to 30 N/mm2 which is approximately 20% of the shear strength of mild steel. As an indication of torque capacity a 100% bond area between a steel 16 mm diameter shaft and a 25mm long bossed sprocket will have a theoretical torque capacity of 221 to 302 Nm. Appropriate generous design margins are needed in practice however - even with these margins working torques of 30 to 40 Nm should be quite achievable on our 16 shaft. As a quick comparison it is worth noting that the rated torque output of a 1000W, 3500rpm electric motor is a touch over 3 Nm.

For the pin/hole fit I've found diametral clearances of 0.1 mm to 0.15 mm work well although I've used larger gap sizes and still made functional joints. To remove a bonded component from a shaft it is usually necessary to heat the joint to 150 C or above to weaken the adhesive. The part can then be knocked off the shaft (or vica versa). If possible use fresh adhesive - my experience of using out-of-date- adhesive is that the set time gets longer and longer although apparently strong (ie haven't failed yet) joints still appear to be produced.

Thread Locking - A further application of cyanoacrylates is for thread locking. Loctite and several other instant adhesive manufacturers produce thread locking formulations (see image right) for retaining nuts on bolts/screws etc and are very useful where secure connections are important such as on steering, braking and wheel securing situations. You may also be interested in anerobic adhesives in these situations - ie mainly liquid adhesives which set in the absence of oxygen.


 

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