Hammer blow
Encyclopedia
Hammer blow, in rail terminology
, refers to the vertical force
s transferred to the track
by the driving wheel
s of a steam locomotive
and some diesel locomotives. The largest proportion of this is due to the unbalanced reciprocating motion, although the piston thrusts also contribute a portion to it. It is the result of a compromise made when a locomotive's wheel
s are balanced to off-set reciprocating masses, such as connecting rod
s and piston
s, in order to keep the ride as smooth as possible. However, hammer blow occurs with the downward force of the wheel's balance weight
onto the railway track, with the potential of causing damage. The rails are subjected to an intense and regular pounding, which can in some cases cause damage to the rails or other structures. The forces are also known as dynamic augment.
Therefore, if reciprocating balancing is increased, it may appear that a locomotive is running smoothly, but at a high cost in hammer blow, especially at high speeds.
Locomotives are balanced to their individual cases, especially if several of the same design are constructed (a class). The normal operating speed is ascertained, resulting in each class member being balanced accordingly. Usually the reciprocating weight is balanced between 45 to 50% of total balance weight. High speed locomotives were the key recipients of such treatment due to the increased level of forces involved, and the exponential factor of stress on the track at these higher speeds.
s of a locomotive
can be completely balanced by weights on the driving wheels since their motion is completely rotational, the reciprocating motions of the pistons, piston rods, main rods and valve gear cannot be balanced in this way. A two-cylinder locomotive has its two cranks "quartered" — set at 90° apart — so that the four power strokes of the double-acting pistons are evenly distributed around the cycle and there are no points at which both cylinders are at top or bottom dead center simultaneously.
A four-cylinder locomotive can be completely balanced in the longitudinal and vertical axes, although there are some rocking and twisting motions which can be dealt with in the locomotive's suspension and centering; a three-cylinder locomotive can also be better balanced, but a two-cylinder locomotive only balanced for rotation will surge fore and aft. Additional balance weight — "overbalance" — can be added to damp this, but at the cost of adding vertical forces, hammer blow. This can be extremely damaging to the track, and in extreme cases can actually cause the driving wheels to leave the track entirely.
The heavier the reciprocating machinery, the greater these forces are, and the greater a problem this becomes. Except for a short period early in the twentieth century when balanced compound locomotives were tried, American railroads
were not interested in locomotives with inside cylinders, so the problem of balance could not be solved by adding more cylinders per coupled wheel set. As locomotives got larger and more powerful, their reciprocating machinery had to get stronger and thus heavier, and thus the problems posed by imbalance and hammer blow became more severe. Speed also played a factor, since the forces tend to increase with the square of the wheel rotational speed.
, which spread the driving power over multiple sets of pistons, thus greatly reducing hammer blow. Less successful was the triplex locomotive
.
The Soviet Union used a different solution to hammer blow with their 2-10-4
. The cylinders were placed above the center driving axle. Unlike nearly all steam locomotives, the pistons had rods on both ends which transferred power to the wheels. The idea was to balance the driving forces on the wheels, allowing the counterweights on the wheels to be smaller and reducing "hammer blow" on the track.
The usage of inside cylinders (which was rare in the USA) results in a more stable locomotive and thus reduced hammer blow. Many European tank engines had inside cylinders to reduce the wear and tear on shunting yard tracks from frequent and heavy use. Outside cylinders are easier to maintain, however, and apparently for many US railroads this was considered more important than other considerations. The maintenance costs associated with the nigh-inaccessible inside cylinders on Union Pacific's 4-12-2
locomotives may have hastened their retirement.
Steam turbine locomotives
lack pistons, valve gear and other fore-aft reciprocating components making it possible to balance the wheels and connecting rods to eliminate hammer blow. Steam turbine locomotives were tried by several companies around the world in the 1930s and 1940s (such as the Pennsylvania Railroad
's S2 6-8-6
and the LMS'
Turbomotive
). Whilst many of these turbine locos suffered problems in service (usually excessive fuel consumption and/or poor reliability) they did prove to be free from hammer blow and offered a way of achieving high power outputs and speeds without causing track damage.
Rail terminology
Rail terminology is a form of technical terminology. The difference between the American term railroad and the international term railway is the most obvious difference in rail terminology...
, refers to the vertical force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
s transferred to the track
Rail tracks
The track on a railway or railroad, also known as the permanent way, is the structure consisting of the rails, fasteners, sleepers and ballast , plus the underlying subgrade...
by the driving wheel
Driving wheel
On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...
s of a steam locomotive
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
and some diesel locomotives. The largest proportion of this is due to the unbalanced reciprocating motion, although the piston thrusts also contribute a portion to it. It is the result of a compromise made when a locomotive's wheel
Wheel
A wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...
s are balanced to off-set reciprocating masses, such as connecting rod
Connecting rod
In a reciprocating piston engine, the connecting rod or conrod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts linear motion into rotating motion....
s and piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
s, in order to keep the ride as smooth as possible. However, hammer blow occurs with the downward force of the wheel's balance weight
Weight
In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...
onto the railway track, with the potential of causing damage. The rails are subjected to an intense and regular pounding, which can in some cases cause damage to the rails or other structures. The forces are also known as dynamic augment.
Principles
The aforementioned reciprocating balance on the wheels attempts to prevent the worst of the reciprocating forces at play on a locomotive, though as a compromise, this extra mass on the wheel causes it to be out of balance vertically, therefore creating hammer blow.Therefore, if reciprocating balancing is increased, it may appear that a locomotive is running smoothly, but at a high cost in hammer blow, especially at high speeds.
Locomotives are balanced to their individual cases, especially if several of the same design are constructed (a class). The normal operating speed is ascertained, resulting in each class member being balanced accordingly. Usually the reciprocating weight is balanced between 45 to 50% of total balance weight. High speed locomotives were the key recipients of such treatment due to the increased level of forces involved, and the exponential factor of stress on the track at these higher speeds.
Causes
Hammer blow is caused by the uneven application of power by a reciprocating piston to rotating wheels. While the coupling rodCoupling rod
right|thumb|connecting rod and coupling rods attached to a small locomotive driving wheelA coupling rod or side rod connects the driving wheels of a locomotive. Steam locomotives in particular usually have them, but some diesel and electric locomotives, especially older ones and shunters, also have...
s of a locomotive
Locomotive
A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...
can be completely balanced by weights on the driving wheels since their motion is completely rotational, the reciprocating motions of the pistons, piston rods, main rods and valve gear cannot be balanced in this way. A two-cylinder locomotive has its two cranks "quartered" — set at 90° apart — so that the four power strokes of the double-acting pistons are evenly distributed around the cycle and there are no points at which both cylinders are at top or bottom dead center simultaneously.
A four-cylinder locomotive can be completely balanced in the longitudinal and vertical axes, although there are some rocking and twisting motions which can be dealt with in the locomotive's suspension and centering; a three-cylinder locomotive can also be better balanced, but a two-cylinder locomotive only balanced for rotation will surge fore and aft. Additional balance weight — "overbalance" — can be added to damp this, but at the cost of adding vertical forces, hammer blow. This can be extremely damaging to the track, and in extreme cases can actually cause the driving wheels to leave the track entirely.
The heavier the reciprocating machinery, the greater these forces are, and the greater a problem this becomes. Except for a short period early in the twentieth century when balanced compound locomotives were tried, American railroads
Rail transport in the United States
Presently, most rail transport in the United States is based on freight train shipments. The U.S. rail industry has experienced repeated convulsions due to changing U.S. economic needs and the rise of automobile, bus, and air transport....
were not interested in locomotives with inside cylinders, so the problem of balance could not be solved by adding more cylinders per coupled wheel set. As locomotives got larger and more powerful, their reciprocating machinery had to get stronger and thus heavier, and thus the problems posed by imbalance and hammer blow became more severe. Speed also played a factor, since the forces tend to increase with the square of the wheel rotational speed.
Solutions
One solution to this was the duplex locomotiveDuplex locomotive
A duplex locomotive is a steam locomotive that divides the driving force on its wheels by using two pairs of cylinders rigidly mounted to a single locomotive frame; it is not an articulated locomotive...
, which spread the driving power over multiple sets of pistons, thus greatly reducing hammer blow. Less successful was the triplex locomotive
Triplex (locomotive)
A Triplex locomotive is a steam locomotive that divides the driving force on its wheels by using three pairs of cylinders rigidly mounted to a single locomotive frame. Inevitably any such locomotive will be articulated...
.
The Soviet Union used a different solution to hammer blow with their 2-10-4
Soviet opposed-piston 2-10-4
Built in 1949, this was one of a number of Soviet locomotive designs that was very successful. The cylinders were placed above the center driving axle. Unlike nearly all steam locomotives, the pistons had rods on both ends which transferred power to the wheels...
. The cylinders were placed above the center driving axle. Unlike nearly all steam locomotives, the pistons had rods on both ends which transferred power to the wheels. The idea was to balance the driving forces on the wheels, allowing the counterweights on the wheels to be smaller and reducing "hammer blow" on the track.
The usage of inside cylinders (which was rare in the USA) results in a more stable locomotive and thus reduced hammer blow. Many European tank engines had inside cylinders to reduce the wear and tear on shunting yard tracks from frequent and heavy use. Outside cylinders are easier to maintain, however, and apparently for many US railroads this was considered more important than other considerations. The maintenance costs associated with the nigh-inaccessible inside cylinders on Union Pacific's 4-12-2
4-12-2
Under the Whyte notation for the classification of steam locomotives, 4-12-2 represents the wheel arrangement of four leading wheels, twelve coupled driving wheels, and two trailing wheels.Other equivalent classifications are:...
locomotives may have hastened their retirement.
Steam turbine locomotives
Steam turbine locomotive
A steam turbine locomotive is a steam locomotive which transmits steam power to the wheels via a steam turbine. Numerous attempts at this type of locomotive were made, mostly without success...
lack pistons, valve gear and other fore-aft reciprocating components making it possible to balance the wheels and connecting rods to eliminate hammer blow. Steam turbine locomotives were tried by several companies around the world in the 1930s and 1940s (such as the Pennsylvania Railroad
Pennsylvania Railroad
The Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....
's S2 6-8-6
PRR S2
The Pennsylvania Railroad's class S2 was a steam turbine locomotive. Only one prototype was built, #6200, delivered in 1944. The S2 was the sole example of the 6-8-6 wheel arrangement in the Whyte notation, featuring a six-wheel leading truck, eight driving wheels, and a six-wheel trailing truck...
and the LMS'
London, Midland and Scottish Railway
The London Midland and Scottish Railway was a British railway company. It was formed on 1 January 1923 under the Railways Act of 1921, which required the grouping of over 120 separate railway companies into just four...
Turbomotive
LMS Turbomotive
The Turbomotive was a modified Princess Royal Class steam locomotive designed by William Stanier and built by the London, Midland and Scottish Railway in 1935. It used turbines instead of cylinders...
). Whilst many of these turbine locos suffered problems in service (usually excessive fuel consumption and/or poor reliability) they did prove to be free from hammer blow and offered a way of achieving high power outputs and speeds without causing track damage.