How Does A Hydraulic Post Pounder Work?

We are entering the spring season, where we receive tons of calls from customers inquiring about hydraulic post pounders for their skid steer or mini excavator.

Many of our customers begin by asking: How do hydraulic post pounders work? What does “energy class” mean when comparing post pounders? And how much maintenance is there?

In this article, I’m going to answer all of your questions, and if you want to know more at the end, just give me a call!

Key Takeaways

- Vibratory hydraulic post pounders use your skid steer or excavator's auxiliary hydraulic fluid to push a piston upwards against a nitrogen chamber.

- When the piston is in the top position, a mechanical valve releases the hydraulic pressure, allowing the nitrogen charge to force the piston down.

- A post pounder's energy class is a measure of the force it exerts with each blow, expressed in foot-pounds.

- Most post pounder maintenance is limited to regular greasing and an annual nitrogen top-up.

What is Happening Inside a Post Pounder?

Vibratory hydraulic post pounders use hydraulic fluid from your skid steer or excavator's auxiliary hydraulic line in combination with nitrogen back pressure to create the hammering action on an internal piston.

The internal piston strikes your post driver tool, which transfers energy to your fence post.

Hydraulic Pressure VS Nitrogen Pressure

Hydraulic fluid is non-compressible, while nitrogen gas is. Therefore, the post pounder uses hydraulic fluid to lift the piston and nitrogen gas to hammer it back down.

Hydraulic fluid is sent from your skid steer or excavator to your post pounder through your auxiliary hydraulic pump and hydraulic lines.

When the piston is at the bottom of its stroke, an internal mechanical valve directs hydraulic fluid into a chamber that will raise the piston.

Once the internal piston reaches the top of its stroke, it is pushing against a chamber filled with nitrogen gas. The pressure against the nitrogen chamber is what is going to create the tremendous downforce once the hydraulic pressure is released.

With the piston at the top of its stroke, forcing itself against a nitrogen chamber, the mechanical control valve releases the hydraulic pressure, and the piston is slammed down.

A Note Regarding Nitrogen

It comes up in conversation once in a while that there are people out there who think nitrogen is used in post pounders only because it is readily available at most welding shops. This is not the case!

Nitrogen gas is used for back pressure because it is non-flammable, inert, and stable. Putting any other gas, including compressed air, into the nitrogen chamber can be very dangerous - so don’t do it.

Other Post Pounder Components

Vibratory hydraulic post pounders also have an internal accumulator. Inside the post pounder, hydraulic fluid is constantly cycling between extreme pressure and no pressure and therefore needs a means to dampen the internal hydraulic forces.

The hydraulic accumulator is a small bladder that applies a small amount of pressure against the regular flow of the hydraulic system. When there is a sudden increase in pressure, the accumulator will fill with hydraulic fluid and absorb the shock.

Because the accumulator is slightly pressurized, when there is a sudden decrease in hydraulic pressure, the accumulator will push fluid back into the hydraulic system, dampening the loss of pressure.

What Does “Energy Class” Mean for Post Drivers?

Energy class refers to how many foot-pounds of force your vibratory post pounder is applying. It’s an easy way of comparing the force of one post pounder against another.

For example, a Montana 750R post pounder has an energy class of 750. This means that this post pounder will apply 750 foot-pounds of force on every blow. When you consider that it is providing 500 to 900 blows per minute, this is a ton of force.

*Foot-pounds of energy are defined as the amount of force needed to move an object one foot. So in the above example, the Montana 750R can move a 750-pound object one foot on every stroke.

What Maintenance is Needed for Hydraulic Post Pounders?

With very few exceptions, the hydraulic post pounder is one of the easiest attachments to maintain.

Aside from ensuring that all hoses and connections are tight, there are only two regular maintenance items needed.

Greasing

Post pounders create a lot of heat when they are operating, and therefore they require regular greasing to help with cooling and lubrication.

You’ll find a grease zerk located on the main body of the attachment, which will feed grease internally. Follow the manufacturer's instructions on the type of grease and frequency of greasing.

Using improper grease may cause the grease to heat up and run down the bottom of the tool. If you notice this, stop pounding immediately and investigate the cause.

Nitrogen Filling

Eventually, you’ll have to refill the nitrogen charge on your vibratory post pounder.

Manufacturers may not provide a recommended refilling interval.

If you begin to notice that your post driver doesn’t have the same power as it used to, it might be time to recharge your nitrogen.

If you suspect your nitrogen is low, you can buy a recharge kit from your dealer. It’s important to note that the nitrogen bottle will arrive empty as it is illegal to ship full bottles in the United States.

You can fill your empty nitrogen bottle at a welding shop and then easily fill your post pounder when your back at the shop.

Conclusion

Vibratory hydraulic post pounders are simple hydraulic attachments that don’t require electrical hookup or high-flow hydraulics.

The simple hydraulic operating principles only require as little as 8 gallons of hydraulic flow per minute, making this attachment usable on nearly any skid steer or excavator.

With only simple greasing every few hours, you’ll be able to pound posts like a pro.