washer for lock washers

Use a lock and flat washer that are the same nominal size as your fastener. For example, if you are using a 3/8" bolt, use a 3/8" lock washer and a 3/8" flat washer. The individual component specs ensure that they will work together just fine. Personally, I like to use the SAE series of flat washers instead of the USS series, but that is only a personal preference.

CSM Engineer is correct. There is a bit more to it though. Just like bolts and cap screws, washers are sold by grade as well. Be sure the washers are of the same grade as the fastener. If the flat washer is to support a lock washer over a slotted hole you may want to opt for a thicker flat washer.

And in addition to the previous two posts, you may find that you need to account for spacing due to washer thickness - the effect is the same as adding a shim. Could be an 'ooops' situation...

The other posters are correct with regard to sizing, grades, etc. However, you don't indicate the purpose of both flat and lockwashers nor the type of lockwasher employed.

If the lockwasher contemplated is a conventional helical spring (or split) lockwasher and the objective is a vibration and cold flow resistant joint, there is little value in using the lockwasher. Regardless of material, a helical spring lockwasher provides little more vibration or cold flow resistance than a flat washer. It's essentially just a flat washer with a split under load. That can be proven by comparing the flattening load to the tensile load developed in the bolt. It takes very little load to flatten a spring lockwasher (typically only 30 - 40 lbf for a 3/8").

Shakeproof, or internal and external toothed lockwashers are slightly more effective because they require several times more back-off torque than a spring lockwasher.

However, the most resistant to cold flow and vibration is a Belleville or conical spring washer. Either single or multiple washers of the appropriate material and characteristics can be used to obtain the desired load and deflection to maintain a spring loaded bolted joint. A flat washer is typically used with the Bellevilles to bridge annular gaps and provide an appropriate bearing surface.

I don't use flat washers with lock washers because it's the bolt I want to keep from turning not the flat washer.

The flat washer is only needed when the surface of the part needs to be protected or the part surface will not support the lockwasher. Some cases for this are mounting rubber products such as mud flaps, or mounting over slotted holes. I was told years ago by a vendor that split lock washers were a waste of time and money. His claim is that they just do not work well. I have to agree to a point. But I can tell you from experience that in fasteners over 1/2" I have destroyed this type of lock washer when unscrewing the bolt because the lock washer gripped so well. In many cases lock nuts or jam nuts are a better choice.

The flat washer is only needed when the surface of the part needs to be protected or the part surface will not support the lockwasher.

When the part needs protection another design of fastener locking should be employed; example:

Some cases for this are mounting rubber products such as mud flaps, or mounting over slotted holes.

After the rubber deteriorate the split washer is useless and better retention can be had with correct design for application.

Agreed. We did not have much of a problem with the rubber deteriorating. The mud flaps we were installing would be changed out at least three times each year. Often due to drivers backing up with the bed raised. This would catch the flap under the wheels and rip them off. The bean counter decided it was cheaper to use this arrangement instead of lock nuts. Besides, when the product deteriorates enough for the fastener to fail it is time to change the part anyway.

Ha! We peen the bolt end slightly too!