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Tech Tip #96: ZF/Hurth Marine Transmissions: A Six Step Program

Helpful tips to keep your ZF/Hurth Transmission running.

Hurth gears have a bad reputation. We had a call last week from a customer in Woolwich, ME who owns a Hurth HBW 100 transmission. Seems he wanted to order another one. His third in 12 years. In the course of the conversation he complained that his ZF/Hurth marine gear was costing him somewhere around $1 an hour to operate. He said that ZF should have a consumer warning label on all small ZF/Hurth HBW mechanical gears: “Hurth the $1 An Hour Gear.”

This got me thinking. Sure to keep the weight down, ZF/Hurth uses lightweight aluminum cases. Like everything else in life, the Hurth aluminum case doesn’t like being submerged in salt water. Additionally, like the air cooled VW bug you drove in grad school, Hurth marine gears have a tiny oil sump, no oil filter, and, usually, no oil cooler. As a result, Hurth marine gears don’t last if run while even just pint low on lube oil. Yes, if abused the seals will leak and the clutches will slip. But having to reseal a Hurth every two years or to buy a new Hurth transmission every 4 years really shouldn’t be necessary with good care.

Here are some easy steps you can take to prolong the life of your Hurth marine transmission. You might want to call this the Foley Six Step Program for Hurth Transmissions! These steps build on earlier Tech Tips we have published. For more information, please read the following  Tech Tips:

1. A well lubricated Hurth transmission is a happy Hurth transmission

You should run the correct lube oil in a Hurth mechanical gear. Upgrade to our Foley Hurth Hytork fluid and your Hurth transmission will last longer. For more information regarding the fluid please read Tech Tip #2, Marine Transmission Fluid. Regardless of the fluid you use, change it frequently and dispose of the old oil properly. For help and ideas in taking care of your old oil please read our Tech Tip #33,Disposing of Used Oil.

2. A cool Hurth transmission is a happy Hurth transmission

Most Hurth gears don’t have oil coolers. You should install an oil cooler in your Hurthif you don’t have one. An oil cooler will increase the torque capacity of your Hurth by one model number because you will have cooler fluid. For example, if you are running a Hurth HBW 100 behind a 50 HP Perkins Model 4108 you are right on the ragged edge of the gear’s capability. Translation: you will be buying a new Hurth gear every few years. However, if you install an oil cooler on your Hurth you will effectively raise the capability of your Hurth HBW 100 to that of a HBW 150. (This is like a free lunch. Where else can you upgrade one model for simply the price of an accessory? Would your friendly BMW dealer give you a new 7 Series for the price of a 5 Series plus a option?)

We used to supply aluminum oil coolers from a German manufacturer for Hurth transmissions. Unfortunately, they wouldn’t hold up in the salt water environment that the Hurth marine transmission lives in. Foley now offers new stainless Hurth oil coolers that we custom build here in Worcester for the same price as an aluminum one. Our stainless steel Hurth oil cooler is a small investment that yields big savings. For more information regarding Hurth transmission coolers please read Tech Tip #54, Marine Coolers and Couplings.

3. Replace your Hurth’s damper plate/clutch

Every time you have your Hurth removed for inspection or new seals, change your damper plate. No matter how good it looks. If your damper plate fails, it will damage the input shaft on your Hurth without you realizing it until it is too late. This little clutch-like plate is inexpensive but does a big job and is time consuming to change if you are not already servicing the marine gear. To learn more about damper plates please take a look at  Tech Tip #59, Coupler or Damper Plate 101.

4. Install a flex coupling to protect your Hurth

No matter how good a sailor you are or how few sand bars and rocks there are where you sail, you should install a flex coupling between the Hurth output flange and the propeller shaft coupling. Flex couplings are often called drive savers and do two things. They protect your transmission from shocks in the event of a grounding or wrapping pot warp around your propeller. They also will help cover-up alignment problems.

We sell both the original red “drive saver” type coupling as well as a newer style coupling that we custom make. The newer style coupling protects your Hurth in two additional ways. First, it offers the same protection as the original red coupling, but because it doesn’t fully break the connection between the Hurth and the prop shaft it allows you to still motor home after an incident. Additionally, these new style couplings install within not between the Hurth output flange the prop shaft coupling so they don’t push your propeller shaft back the same 1.5+” as the red style couplings do. We’ve been able to keep the cost of this newer style flex coupling down to the same level as the style red coupling. The choice is yours.

5. Check and recheck your shaft alignment with a dial indicator

While a flex coupling will mitigate bad alignment, it can’t do the whole job. Check also your engine mounts. Worn mounts will cause the prop shaft to move around and can damage your Hurth.

6. Include your Hurth in your daily checks

When you do your dailies, include your Hurth gear. Check the level of the lube oil. Look for leaks. Look at the hoses going to the oil cooler. Is your Hurth happy?

There you have it. Our Six Step Program to avoid having your Hurth becoming a $1 an hour marine transmission.