Professor Morris in the laboratory. Photo: Joe Bonnello
This Month's Lesson: Inside the Saginaw Box
Welcome to the 22nd class in the Dirt Sports University Series, and the third on power steering. We finished up our second class in power steering with a look into the trio of Terrible Herbst prerunner trucks that were built almost 20 years ago yet incorporate components that are still leading-edge today. In this month’s class we will look into what makes the Saginaw power steering gear box work, and the particular areas you might need to set up a little differently to optimize the Saginaw steering box for your particular vehicle application. Today’s class will start with the Saginaw box input shaft being turned and carry through to the steering arm that moves the tie rods. Going forward in our next classes we will move into rack-and-pinion systems, power steering pumps and all the other power steering system components that are used and needed in today’s off-road vehicles.
The servo valve has the key role of controlling both the oil flow and direction. WHAT MAKES IT TURN: From the moment you place your hands on the steering wheel there are many more components in play to connect that nicely padded wheel to the tires that touch the ground and are just waiting to shred some dirt for you. Once you turn the steering wheel and start putting in motion the steering shaft and U-joints to point your vehicle in any direction, the first component that actually does something is the servo valve assembly. For just about any common steering system being used today, there will be a servo valve to direct the high-pressure hydraulic oil that comes from the power steering pump to the proper side of the piston inside of the steering ram. While this hydraulic flow of oil through the servo and its function might seem quite simple, there have been untold hours involved by many factory engineers to get it to work. The basic operating components are the barrel valve that directs the amount of oil flow and the direction that you want to turn, and the torsion bar that gives the feel to the steering. Between these two items they provide the feel and proper balance that get the steering system in motion whenever the wheel is moved. The servo valve assembly functions the same whether it is an integral part of the Saginaw steering box or a standalone unit incorporated into the steering shaft on its way to a rack-and-pinion steering unit. The nice aluminum housing in the picture below is made by Sweet Manufacturing to house the Saginaw servo valve that would normally fit with the Saginaw steering gear box assembly. There have been other smaller steering servos through the years that are available to use. These small versions really only have a place in lightweight off-road cars where you are trying to save space and money. The good, old larger Saginaw servo valve has proven itself year after year as having a better bandwidth for precise oil flow control, which gives better overall steering response and the confidence to the driver that is necessary to go fast.
The servo valve torsion bar plays a critical role in steering feel. THE TORSION BAR: The name is just what it is, a torsion bar. It provides a spring force that gives you feel at the steering wheel that has its diameter and spring rate properly matched against the hydraulic oil flow and pressure to aim your vehicle comfortably in the right direction through the entire engine rpm range, steering turning angle and vehicle speed. The diameter of the torsion bar is used as a tuning aid to give a good firm and comfortable feel at the steering wheel. There are several diameters available for the torsion bar, as well as custom sizes that can be ground as needed for special applications. Using a small diameter torsion bar, or what would be called “light,” can work well for you provided that you do not get carried away. With going too light you will have no feel for what the front tires are doing or where they are aimed, and you will tend to chase the steering wheel around and over-steer the vehicle constantly. Being able to steer with one finger is not the right setup. If the torsion bar is too large in diameter, then you will feel like you have to wrestle the vehicle around too much with a heavy-feeling steering wheel. A good compromise must be made in torsion bar size between good road feel and precise steering versus having the steering too light or heavy. Many times several size torsion bars will need to be driven with and tested in your vehicle to determine the correct size that will work well to match the entire steering system. The proper steering feel needs to take into account the steering linkage motion ratio, tire size, steering arm length and suspension geometry. No two vehicles are really the same unless they are exactly the same — and I mean exactly. There is no magic of being able to copy what you see on another vehicle and expect it to be perfectly matched to your vehicle. This assumes that the rest of your steering system is set up properly. There are times that other items in the system can make the steering act like this. You should not be using the torsion bar diameter choice as a band-aid for other parts of the steering system that are not properly matched to the entire system. Some problems are due to the wrong power steering pump speed, flow and or pressure, hose sizes and restrictions. We will get to these challenges in a later class.
The barrel valve directs the oil flow. THE BARREL VALVE: This is where the business end of directing high-pressure hydraulic oil to the steering ram happens. The precision-ground slots on outside of the barrel valve work in conjunction with similar slots on the inside of the outer drum. Opposing slots are normally not aligned to provide directional steering of oil flow, and the torsion bar is what keeps the barrel valve slots in a neutral position. During this neutral time the oil flow is merely reticulating in the system. The moment that you move the steering wheel and start to move these slots off their neutral zone and into a directional steering flow path, be it left or right, then oil flow is directed to one side or the other of the steering ram piston. At this point the vehicle steering system powers itself to go the direction that you are turning the steering wheel. This is a simple version of what happens with all of the steering components working in unison. As you demand more steering capacity with larger vehicles and tire sizes, the steering oil flow needs to increase. As you go to larger power steering pumps and the necessary larger oil hose sizes, all of the oil holes in the barrel valve assembly will need to be increased in size so as not to create a flow restriction. This is not something you are going to do with your drill at home. Please leave this modification to the experts. Take a moment and look for the small 3/32-inch drive pin on the upper inside edge of the larger part of the barrel valve (below). This pin aligns with a similar-sized slot at the base of the torsion bar housing. At the end of the day this small pin is all that connects the steering wheel to the front tires. The real work is done with the hydraulic oil flow and pressure. As your vehicle engine is off and you want to muscle it around, the only thing doing the work to steer the front tires is the torsion bar and that little pin to take all of your steering effort. Fortunately, there are a pair of large tangs that run out of travel in slots at the base of the servo valve, thus limiting excess damaging load on the torsion bar and the small 3/32-inch drive pin. You can have a second drive pin added to limit the possibility of breaking the single pin. Through the years I have had this small drive pin shear many times, and have added a second pin on the opposite side for safety and reliability. This was another modification pioneered early on by Tom Lee.
The steering ratio starts here. RECIPROCATING BALL DRIVE: This is what determines the gear ratio of the steering box. While there is a difference in the three gear teeth on the sector shaft between a straight ratio and a variable ratio steering box, the number of worm gear ball groves in the ball shaft is what really matters. The ball bearing groove width, along with a ball count of 22 to 24, will vary between the different ratios. The funny looking curved tube bolted to the top of the combination of the ball screw housing, ram piston, sector shaft drive teeth and around what looks like a large hunk of roundish steel is to keep all of the 20-some ball bearings in motion and circulation within the ball screw steering ratio drive system. It might sound complex, but it’s not. With the ball screw drive you have a straight ratio choice of 12:1, 14:1 and 17.5:1. While there are two choices of variable ratios available, we are going to pass them by as viable options due to an odd steering play that is inherent when using different size drive teeth at the same time. And, no you cannot mix and match these parts to come up with your own special ratio, as they are only able to be used as a matched set.
A ratio chart shows various options. SECTOR SHAFT TOOTH STYLE: Take a look at the picture of the two sector shafts and their ram pistons that incorporate the matching teeth. There are two things going on in the picture (below). One is that we wanted to show you what the straight tooth drive looks like on the left side, compared to the variable tooth drive on the right. Notice the straight ratio provides three evenly cut teeth that do the full range of steering. Now look at the right set that has a much larger center tooth with small teeth alongside. This is where the issue starts with keeping a good combined tooth engagement when trying to use the variable ratio. The concept is good, but unless you are planning to use it in your street hot rod, the application for heavy off-road use is very slim. The second part to take notice of in this picture is the two different ram diameter sizes that are available with certain steering box ratios. The larger diameter ram has quite a bit of power assist available from it. The smaller ram diameter usually needs an auxiliary assist steering ram used with it. GEAR RATIO CHART: Get with your favorite steering gear box supplier or, better yet, take a look at what the leaders in the off-road industry have to offer for your particular vehicle. A gear ratio chart for the Saginaw steering gear box will show you what combination of ball drive ratios are available with the sector shaft drive tooth combination that best fits your application. IN CLOSING: Look at the top competitors to see what they are winning races or events with. Chances are that they are using a completely matched system of components from the same steering company. Be smart and let the steering experts do what they do. You just need to drive your off-road dream and come up with the money to have someone else to do the hard work. Do your best to stay away from mixing up your own combination of components unless you are some form of brilliant fluid and hydraulics engineer. Even if you are a brainiac, odds are that a real good, full product line steering company will have way more collective experience on the subject than you possibly can. If the steering company that you are working with or thinking of going to does not have or offer a complete solution to all of your power steering needs and in your desired area of use, then look elsewhere. Remember that at the end of the day, you and you alone are the one in control and responsible for your choices. Ideas and thoughts of any kind that we may put in your head are your responsibility to investigate properly how you will apply and use them safely. Be responsible and make your decisions wisely on any project you are going to undertake. HOMEWORK: If you are not able to get out to the races where the big boys play, take the time to look closely at the pictures here in Dirt Sports that are of the top competitors. There is also good information to be had in looking at the Masterpiece in Metal vehicles for ideas. If you do not have a subscription to Dirt Sports yet, then step up and get one so that you don’t get left behind in class. You can also get back issues sent to you, as well as electronic subscriptions. Class Dismissed. Professor Tom Morris Dirt Sports University and School of Hard Knocks