Other Articles - December - 2018

Bushings: How Critical Can They Be?

Like many components used in the transmission industry, the term bushing can be described using various other names. A plain bearing, sliding bearing, solid bearing, or friction bearing are all terms used to describe what many of us refer to as a bushing.

Bushings, like bearings, have been around longer than the automobile and are used in various industries. Bushings are compact, lightweight, have high load-carrying capacities, and are the least expensive form of bearing in use today. A bushing will act as a bearing if it’s inserted into a housing or bore to provide a surface for a shaft that’s rotating.

A linear bushing is typically pressed into a bore and provides support for a shaft that moves fore and aft (in/ out). If a bushing isn’t press fit, it’ll be retained with a dowel or snap ring to hold it in position.

Bushing Design

From a design standpoint, why do engineers choose to use a bushing in some locations and a bearing in others? Why do we see some updated components replacing a bearing with a bushing or a bushing with a bearing?

It’s safe to say that the reason we use bushings and bearings in transmissions is primarily to reduce friction and wear and to maintain component alignment. Whether we’re talking about a bearing or a bushing, each has its own advantages and disadvantages.

Bushings are usually chosen over a bearing for one of these reasons:

  • Bearings are typically 25-400% more expensive than a bushing.
  • The tooling needed at the manufacturing level to control the bearing bore dimensions and install the bearings into the bore is 50-75% more expensive than the tooling used for a bushing.
  • Bushings tend to be 50% lighter than a comparable needle bearing and 1/14th that of a comparable ball bearing.
  • A bushing is much less sensitive to shock loads or oscillations; bearings can develop brinelling, leading to bearing race damage and ultimately bearing and seal failure.
  • Bushings can tolerate much greater shaft misalignment.
  • A bushing provides a much larger contact area than a bearing, spreading the load across a larger area.

Three types of bushing designs are typically used in automotive applications:

  1. Clinched (Puzzle Piece Design) bushing: This is a sintered-material, layered bushing and currently the most common type in use today. Several OEMs use this type of bushing. After installation, the OEM may bore the bushing to make sure they work efficiently. This process is called “machine in place” and is used on some transmissions to create a true bore after bore machining and bushing installation.
  2. Solid (No Seam) bushing: This type of bushing has the tightest wall and OD tolerances in use today. These bushings are manufactured with a layer of babbitt material and with seamless construction, making them easy to install. Additionally, their ridged structure eliminates the machine in place process, which may have been required at the OEM level.
  3. Split (Butt Joint) bushing: Split bushings are generally used for low load or low speed conditions. The most common transmission use for this application is shift linkages, although some suppliers are now making them available to service internal transmission components.

Bushings can be made of a variety of materials, such as bronze, steel-backed babbitt, plastic, nylon, or cast iron. The key to the material design depends on the type of use the bushing will experience. The bushing material must support and protect the component from damage. This means it must be hard enough to support the load but soft enough to protect the shaft moving inside it.

It’s important to understand that a shaft isn’t typically centered in the bushing during rotation.

This offset distance is known as eccentricity, which provides clearance for lubrication as the shaft rotates. This means the bushing provides a very precisely sized bore for the shaft to ride in as it carries the lubricating oil with it. Since this oil clearance is critical to the longevity of the shaft and the bushing, it’s important to use the correct lubricant type and viscosity.

Bushing Materials

  • Steel Backed/Babbitt: These bushings offer great fatigue resistance and load-carrying capacity with excellent surface behavior and resistance to corrosion. It’s a lower cost material than bronze and it offers a great service life. These bushings consist of a steel tube for support, and babbitt, which is spun cast into the tube.
  • Its material properties allow for conformability and embed ability to protect the shaft from damage, such as scoring from contaminates in the lubrication oil or fluid. Additionally, these bushings can be coated with a dry film lubricant or fluoropolymer for intermittent dry starts.
  • Bronze: This is a very durable material, which offers great service for low-speed, high-load, severe-duty type service. There are 17 different types of bronze materials in use today. The two most common bronze materials are:
    • Oilite: an oil-impregnated material that offers good service life for low or high speeds and high or low temperatures. You’ll hear this material called self-lubricating, as its porous walls draw lubricant into the bushing by way of capillary action and release it when heat and pressure are applied. This type of bushing is typically manufactured using a sintered or powered metal process.
    • Copper: This type of bushing material typically consists of bronze with tin, aluminum, or silicone added to the mix. It offers good corrosion resistance, so it’s the choice for marine use and other hostile environments.
  • Cast Iron: These bushings are typically used to support a hardened steel shaft. The friction coefficient is very low so the cast iron “glazes over” during use, leading to very little wear and very long life.
  • Plastics/Nylon: Look for these bushings in dry applications, such as transmission shift linkages, and in some wet applications, such as shaft supports in some transmissions. Common materials include Nylon, Teflon, Urethane and Vespel.

Bushing Features

Coatings: Sometimes you’ll find a bushing has an internal coating on the surface that contacts the shaft. The material is placed on the bushing after it’s manufactured. This process is called thermal spraying. The materials used vary, depending on the operational requirements of the bushing.

The most common coatings are dry film lubricant, PTFE (Teflon) or graphite. Coatings are designed to increase the life of the components by reducing the friction between the bushing and the shaft it’s supporting.

Grooves: Some bushings have oil channels, holes, or grooves. This feature allows lubricating oil to flow across the busing, making full contact with the shaft it supports.

Indents: Small pockets are machined into the bushing surface to retain oil on the surface of the bushing.

Bushing Damage

Pitting: Sometimes you’ll see pitting on the bushing surface. This damage is typically due to lubricant cavitation, but not the type of cavitation you’re used to dealing with in the transmission world. Lube cavitation occurs when the oil film thickness breaks down, leading to hot spots on the bushing. The resulting hot spot causes the dissolved air bubbles in the fluid to implode, leading to pitting. One of the most common causes of pitting is incorrect type or viscosity oil.

Wear/Scoring: Scoring damage occurs when the lubrication barrier breaks down, allowing the shaft and bushing to contact each other. If your bushing shows excessive scoring, the issue could be due to improper clearance between the shaft and bushing, but it’s more likely caused by a lack of lube flow. If you find scored bushings, inspect the lube flow path to the bushing. You’ll likely find restrictions or contamination in the circuit, which could account for the failure.

Bushing damage may appear differently, depending on its cause:

  • The whole bushing surface shows evidence of damage: This is typically due to lack of lubrication, the wrong lubricating fluid, or incorrect bushing-to-shaft clearance. The bushing failure (see photo page 18) caused P0741 to set on a 6L50.
  • The bushing may show evidence of wear only on one side: This usually indicates the shaft was side-loaded during operation. With this type of failure, you’ll need to inspect the components below and above the failed bushing to determine the cause. The bushing failure caused a 3-2 bindup on 6F35/6T40 applications.

Ground Wires: Transmission ground wires, along with most other grounds on the car, are critical for the vehicle to function. All power and ground connections must be clean and free of excess resistance to prevent a long list of problems, including bushing failure.

Installation

When installing a bushing, it’s essential to use the correct bushing driver or installer. The installer must be the correct size and its surface must not damage the bushing. Most techs lightly lube the area of the bushing in contact with the installer prior to installation.

When installing the bushing, some techs will drive the bushing in place while others prefer a press. Where possible, the arbor press installation is the preferred method, as it reduces the chance of bushing damage.

As you can see, that simple little bushing is critical to the longevity of your transmission repair. Bushings are critical to shaft and housing alignment, which is critical to seal alignment and ultimately the life of your clutches and gear train. This is where you need to pay attention, as it’ll pay dividends for reducing comebacks and improving customer satisfaction.

Special thanks to the engineering staffs at GM and Dura Bond for assistance with this article.

Until next time, remember: “Life is trying things to see if they work.”