Drive Train Components Reflect Increases in Weight of Apparatus
drive-Diagrams courtesy Dana-Spiccr Corp.
Progress in fire apparatus design and performance is essential to keep pace with growing industrial hazards and expanding structural problems. An increase in the equipment carried on fire apparatus to meet the changing conditions is a parallel development. This increase in quantity and weight of equipment and extinguishing agents is reflected in a corresponding increase in the weight and cost of the vehicle and its drive train components.
Changes in drive train components are necessary to provide adequate motive power and load capacity for the drive train units to maintain a proper balance in design strength and to preserve die gross vehicle weight to performance ratio. Chassis engineers give careful attention to the selection of drive train units (engine, clutch, transmission, universal joints, propeller shaft, rear axle, wheels and tire size) as each affects vehicle performance and service life.
Prior to World War II, there were few, if any, drive train problems. Usually only two sizes of gasoline engines were offered by any fire apparatus manufacturer, which simplified the matching of drive train components. But now, the purchaser has an option of many engine sizes, both gasoline and diesel.
More water carried
A pumper is a good example of rising vehicle weight. Water tank capacity has expanded in 20 years from 80 to 500 gallons, the capacity currently specified for most pumpers, for a 525 percent increase in the weight of water alone. The hose load weight has averaged an increase of over 50 percent during this same period. Many other items of equipment are now carried to add to the total weight of the vehicle in service. So, the criterion for the selection of engine power and other drive train components by the fire service is changing.
Until 20 years ago, the rated pump capacity was the controlling factor in engine power selection, but with the increasing weight of apparatus, the selection of engine power to provide acceptable road performance is now determined by the gross vehicle weight. In NFPA Specification No. 19, Section 2520, minimum acceptable performance standards are given.
There are two terms commonly used to express the work capability at some stated crankshaft speed. When the rated pump capacity was a determining factor in engine power selection, the use of horsepower as a measurement of capability worked well. It was easy to balance with the pump requirements, which were usually calculated in terms of water horsepower to indicate the work done by the pump. With the change to vehicle performance as the determining factor in engine power selection, it becomes more important for the fire service to understand torque in evaluating engine performance.
Engineers have used torque values continuously for determining the proper size and capacity of all units in the drive train, based on the net torque of the engine when fully equipped with all operating accessories and the vehicle exhaust system. The performance of die first diesel engines surprised most fire departments and created a genuine enthusiasm. Due to the difference in rating gasoline and diesel engines for horsepower (which should not be so with governed speed engines) the fire service expected some sacrifice in road performance because of the lower horsepower rating of die diesel engine. That the performance is improved in most vehicles is due to the higher torque value. As maximum torque is developed at approximately one half the full load governed speed of the engine and is fairly constant to the full load governed speed, its values are not affected by testing and rating at speeds above full load governed speed.
The relationship and definitions of horsepower and torque are explained in the June 1968 issue of FIRE ENGINEERING, pages 63-65, so they will not be repeated here.
For each commercial truck chassis model suitable for fire service use, the manufacturer lists at least two sizes of gasoline engines and one or two optional diesel engines. For custom-built apparatus, there is a wide choice of power range and type. Among the gasoline engines, Waukesha has two sizes, with die model F 817 G the most popular. Waukesha also has the LeRoi model TH 884, which is a V-8 for high performance. Continental has three engines, with the dual carburetor model S-6820 a strong favorite. Mack has two engines, with the ENF-707-C preferred for best performance. Hercules offers two engines, plus the HallScott 590FE, 6156FE, and 6182FE, with the 6156 FE most in demand.
Among diesel engines, the variety is almost equal to that of gasoline engines. Mack has three sizes in the END series, all turbo-charged for sustained high torque output. Cummins has one diesel naturally aspirated and three turbo-charged to meet any fire service performance requirement. Detroit Diesel also has four sizes to cover the range of power requirements for the fire service.
This is not a summation of total engine availability, but it indicates the wide range of availability. It represents 13 gasoline and 11 diesel engines.
A fire department frequently specifies a certain engine or a minimum size and type acceptable for one or more reasons, but it is not common practice for a fire department to assume the responsibility of specifying the manufacturer, model and rating of other drive train components.
However, there are some exceptions, such as the transmission to be supplied. With the manual shift type of transmission, there is usually a preference for either the constant mesh or the synchromesh. If an automatic transmission is required, the manufacturer and model is usually specified. Information for such specification is usually supplied to the fire department by a fire apparatus manufacturer’s representative who knows the correct specification to match the engine desired by the fire department.
The component manufacturer’s approval of the clutch and transmission for use with a specific engine involves more information about the vehicle than the engine alone. Such information has to be filed with the component manufacturer and includes: maximum net engine torque, type of engine (gasoline, diesel, 2-cycle or 4-cycle), number of cylinders, gross vehicle weight, weight distribution (percent on front and rear axles), rear axle ratio and tire size.
Clutch pedal pressure
It will be noted that more than net engine torque is necessary to determine the suitability of a specific unit for fire service use. For example, a single plate clutch is satisfactory up to a certain capacity. Beyond that point, a two-plate clutch is used. One of the factors determining the maximum torque for a transmission is the clutch pedal pressure. The capacity of a clutch can be increased by the use of stiffer springs, but the spring pressure must be overcome by the driver’s foot pressure. A maximum of 65 pounds applied pressure is considered acceptable for the clutch pedal disengagement. A more acceptable pressure for disengagement is 50 pounds.
High pedal pressures are conducive to a driver riding the clutch, that is, driving while one foot is on the clutch pedal all or most of the time. This is die direct cause of the high rate of clutch wear in many fire departments. High pedal pressure has been cited as the cause of some fire department accidents. High pedal pressure also produces shock loading of the drive train components, as the driver does not make a slow, smooth engagement.
A practice that produces a high rate of clutch wear is starting the vehicle movement with the transmission in second gear.
Hiding the clutch and starting vehicle movement in second gear are eliminated by an automatic transmission. The clutch for starting vehicle movement is replaced by a torque converter, which transmits engine power through hydraulic fluid, precluding shock-loading of drive train components, and eliminates the clutch pedal.
There are two automatic transmissions which are accepted in increasing numbers by the fire service as their load capacity rating covers quite a wide field of application. These transmissions are the Dana-Spicer model 184 and Allison HT-70. Both of these transmissions were described briefly in the January 1969 issue of FIRE ENGINEERING.
Tiiere are some basic points of design difference that will be discussed briefly for a better understanding of the applicability to fire service in your area.
The Dana-Spicer model 184 transmission uses a three-stage torque converter to replace the conventional disk clutch and to replace gearing for torque multiplication. Reference to Figure 1 will show that gears for reverse movement are the only gears used in the drive. The maximum torque multiplication is 5.5:1, which is at the start of movement, sometimes called stall speed.
As the vehicle speed increases, the torque multiplication decreases until at a vehicle speed of approximately 25 mph, the transmission governor locks the drive in “direct,” which cuts out the converter action. From this direct lockup speed to top vehicle speed, the engine drive is in direct. When the vehicle speed decreases to approximately 22 mph, the governor locks in the torque converter and disconnects the direct drive.
Importance of terrain
The important point to consider in selecting an automatic transmission is the kind of terrain your department serves. Can your apparatus negotiate all the streets and highways in direct drive? Stops for traffic lights or stop signs are not considered—only the grade conditions. If the grades are low, permitting direct drive without downshifting, this transmission is your choice.
If you are required to downshift but desire this transmission because of its simplicity, you must be satisfied to negotiate bills and steep grades at lowspeeds. By low speed, we mean less than about 22 mph, the point at which the torque converter is cut-in to multiply engine torque. The speeds of governor cut-in and cut-out are given as approximate, as the governor has some adjustment range to change these points.
The action of the three-stage converter is shown by the schematic diagram, Figure 2, and the parts of the torque converter are identified in Figure 3.
The Allison automatic transmission model HT-70 is shown in Figure 4. The torque converter is a single-stage, three-element type, shown in Figure 5, in which the elements are identified. The torque converter for this model transmission is made with four values of torque multiplication. At stall speed, these are 3.10:1, 2.82:1; 2.55:1 and 3.5:1. Some fire departments make the mistake of specifying the torque multiplication ratio or the series model number for the torque converter. Each application of the HT-70 automatic transmission is checked by Allison engineers, and they specify the proper torque converter to be furnished.
The transmission has a full range of gearing, six speeds forward and one reverse with full power shift in all forward speeds. All gearing is planetary type, so all gears are in constant mesh. Optional sets of gear ratios are available, and like the torque converter, Allison engineers specify the combinations that match the engine torque and characteristics, such as engine speed range, type of engine, terrain over which the vehicle will be operated, vehicle gross weight, axle ratio and tire size. Therefore, other than specifying the model HT-70, no attempt should be made by a fire department to specify the torque ratio or gear ratios.
The next components in the drive train, the universal joints, propeller shaft and rear axle, have an added factor affecting the selection, the low gear ratio of the transmission. These components must be capable of transmitting the full net engine torque multiplied by the low gear ratio of the transmission, except as this may be modified by the skid torque. The skid torque is the torque required to skid or spin the rear tires of the fully loaded vehicle on dry concrete.
Again, a fire department should not specify the model or series for the universal joints and propeller shaft, for example the Dana-Spicer series 1600, 1700 or 1800. All the factors we have cited are considered by the manufacturer in selecting the proper series for a specific match with all the drive train components, vehicle weight and tire size.
The method of rating rear axles has bugged most chassis engineers for fire apparatus for these many years. The exact formula is a closely guarded secret. We do know it is based on skid torque plus an experience factor. The published rating for a specific axle is usually based on commercial truck application with an engine that does not deliver sufficient torque to skid the tires on dry concrete with a fullyloaded vehicle. Thus, the published load capacity rating for most rear axles is not applicable to fire apparatus, as the ratio of powder to weight is much higher for fire apparatus than is usual for commercial trucks.
Rear axle selection
In the construction of fire apparatus, using either a commercial truck chassis or a custom built chassis, the rear axle selected is matched with the net engine torque, vehicle gross weight and tire size.
The rear axle ratio is selected to match the speed range of the engine (to full load governed speed) and the top vehicle speed required, usually 50 mph. The grade requirements may modify the speed desired to obtain a better overall performance. For grades under 10 percent, no modification is usually required. The gear ratio used is seldom the mathematical ratio to exactly match the engine full load governed speed and the top vehicle speed. A reasonably close available ratio is used.
An example of the selection method is as follows:
The full load governed speed of the selected engine is 2,400 rpm. Tire size is 9.00 X 20, 10-ply rating, which has a rolling radius of 19 inches, or 1.58 feet, and makes 523 revolutions per mile.
Engine rpm X time (per hour) = Revolutions per mile of tire X miles per hour
The transmission is assumed to be in direct drive, 1:1 ratio.
Substituting: 2,400 X 60 523 X 50 = 5.50
No rear axle is available with a 5.50:1 ratio. The nearest ratio is 5.286:1, which would be used. This will slightly increase the top vehicle speed, as
2,400 X 60 523 x 5.286 = 52 miles per hour
Our hypothetical truck has a gross vehicle weight of 24,000 pounds, which is distributed 36 percent on the front tires and 64 percent on the rear tires. The load on the rear tires is then 15,360 pounds. At 2,400 rpm, the engine develops a net torque of 550 lb. ft. and at 1,600 rpm, the maximum torque developed is 600 lb. ft. net.
Power to skid tires
The coefficient of adhesion of rubber tires to dry concrete is 0.60. Thus, the weight on the rear tires times the coefficient of adhesion will give the tractive effort necessary to be delivered to the tires on the ground to skid the tires on dry concrete.
15,360 x .6 = 9,216 pounds
If the low gear ratio of the transmission is assumed to be 5.5:1, the total multiplication of the engine torque will be 5.5 X 5.286 = 29.07. With an efficiency through the drive line and gearing of 85 percent, the maximum power delivered to the tires at the ground will be:
600 X 29.07 X .85 1.58 9,383 pounds
We thus have sufficient pou’er to skid the tires so more power is unnecessary.
Finally, we check to see what performance we can expect in the way of gradability in direct drive at 52 mph. The simplified formula we will use neglects some fundamental values, such as wind resistance.
Tractive effort — 550 X 5.286 X 90 1.58 = 1,655.8 pounds 1,655.8 = 12 (30 + 2,000 sine a) — 5.3 percent grade
This would be considered very good performance for top speed.
This is only a brief look at the engineering needed to provide good performance and a reliable drive train.