Help Understanding Rubber Bushing Lubrication please

[YoshiMoshi]

TL;DR Summary

Why/how lubricate a rubber bushing?

Are rubber bushings in the automotive industry designed/engineered to never need lubrication? Doesn't eventually all rubber eventually "dry out" resulting in cracking and stiffening of the rubber? I think that the purpose of rubber bushings is to allow for some movement in the joint to occur.
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Take for example the classic dog bone motor mount with rubber bushings. Does one simply just take a brush with silicone grease on it and wipe it across the two exposed surfaces of the two rubber bushings to help lubricate and soften the rubber? For this type of bushing, the bushing is meant to stay stationary and not rotate.

I know that some people replace rubber bushings with polyurethane bushings. These come in two half and a zerk fitting can be added to inject grease to lubricate the bushing and the space between the bushing and metal container. Is this
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For this type of bushing though, wouldn't the rubber bushing rotate inside of the metal piece as well?

Basically I find it very odd that for rubber bushings you have to use a brush and can only apply grease to the exposed surfaces of the rubber bushing, while polyurethane you can use a grease gun with a zerk fitting to get grease on more than just the two exposed surfaces. Why is this?

Also, I recently discovered red rubber grease. Looking at the details online, it sounds very similar to silicone grease, help lubricates rubber. Which is better at accomplishing this though? Red rubber grease or silicone grease?

 

[Ranger Mike]

like everything in a typical automobile, it is a matter of compromise. The typical family grocery getter is a compromise between economy, comfort and performance. It is a balance between these factors. Suspension components are no different. The main reason rubber bushings are used is they are cheap, easy to install, provide a quite ride and will last at least the life of the car as designed.

The first thing stock car racers do is pull out these bushings and replace them with steel, aluminum or polyurethane bushings. These bushings are constructed from high performance polyurethane, providing stiffer and longer lasting performance than rubber bushings. The metal bushings provide a harsher noisier ride but are the top performers. Rubber bushing yield the quietest ride and performance of Jello!

Front end bushings total 12 pieces. Cost of a rubber bushing - 1$ same with nylon, polyurethane $ 5, aluminum $ 7, steel $ 12 steel. 12 x $ 12$ = $ 144 added to cost of each vehicle to go with steel bushings.

 

[jack action]

Rubber bushings are meant to absorb shocks. They act as a spring so they don't need lubrification. The rubber is often molded with the metal part such that motion is impossible, other than the twisting and squishing of the rubber.

Racing vehicles usually prefer stiffer components, hence replacing rubber bushings with polyurethane bushings (or even metal ones). They prefer to leave the shock absorbing entirely to the vehicle springs and shock absorbers (easier to predict the outcome when modifying stiffness and damping). These polyurethane bushings are thus free to rotate without twisting. And they often squeak. The lubrification helps prevent wear and squeaking.

 

[YoshiMoshi]

Thanks. I think I understand now. Rubber bushings are molded into their metal casings, while polyurethane bushings are typically molded outside of their metal casings. Polyurethane bushings come in two pieces, to allow them to be assembled into their metal casings to be used. Rubber bushings squish and twist, while polyurethane is a little bit harder and will not twist, instead it rotates. The grease in polyurethane bushings is required to lubricate the surface where the polyurethane material rotates against the metal casing. Rubber bushings do not rotate, so the grease is not needed in-between the metal casing and the rubber bushing.

Is there any validity though that with time, rubber "dries out" and "stiffens" with age. The stiffer rubber will not squish or twist as easily. Silicone grease can be added to outer flat surfaces of the rubber bushings to soften it up to allow it to squish and twist more easily, better absorbing shocks and vibrations?

 

[Ranger Mike]

stock bushings are there for a reason. Life of the car. if you want to improve performance or ride, replace them otherwise do not money with them. if you do, you are going to mess
them up and screw up the ride.

 

[jack action]

Polyurethane bushings come in two pieces

They can be one piece. (They usually are.)

Silicone grease can be added to outer flat surfaces of the rubber bushings

I wouldn't do that unless it is recommended by the manufacturer. (I don't think any does.) Trying to select an unknown grease composition for an unknown rubber composition might not give the expected results.

Is there any validity though that with time, rubber "dries out" and "stiffens" with age.

At that point, the rubber usually cracks and it is thus time to change the part.

 

[Baluncore]

Motor vehicles use synthetic nitrile rubber, not natural rubber.
Natural rubber would be rapidly destroyed by absorption of excess fuel and oil. Nitrile rubber is stronger and more resistant to fuel and oil, but it is less flexible.

All polymers 'dry out' by loss of plasticizer from the surface. That tends to reduce flexibility and volume, resulting in tension cracks. Cracks accelerate failure by increasing the surface area exposed to the environment, and by focussing stress deeper, leading to fractures and loss of integrity.

Water is absorbed by all polymers, some more than others. Natural and nitrile rubber are lubricated by water. Keeping polymers wet tends to reduce the loss of plasticizer, partly by reducing surface tension and cracking.

However, polyurethane can be damaged by excess water absorption. As a compromise solution, the surface may be sparingly treated with a silicone oil or grease to reduce the rate of moisture absorption and swelling.

Rubber does not last forever, but then neither does a horse.