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Hydraulic » Hydraulic Article Resources » Cushioning In Hydraulic Cylinders


Cushioning In Hydraulic Cylinders

Danivasa Ramesh
General Manager-R&D, Wipro Fluid Power Limited, Bangalore


Abstract

Hydraulic Cylinders are the 'muscles' of Mobile and Industrial Hydraulic machines. They handle enormous to tiny implements at ease with tremendous velocities. These implements quickly reveres back and both and forth. The gathered momentum and rate of change of momentum causes huge impact on the end covers. This is detrimental to cylinders as well as to equipment and operator comfort. So there is a need to "soften the blow".

The locked up oil just before the end butting stroke is allowed through a "carefully designed flow resistance path'. This produces the back up pressure hence opposing braking. This force decelerates the motion and result in a smooth butting. This is termed as "Cushioning in Hydraulic Cylinders".

There are many problem associated with cushioning since the comfort of cushioning is assessed by the fineness of feeling of operator. Hydraulic and motion parameters relevant to cushioning are
  • Smooth change of velocity
  • have restriction leading to 'hydraulic shock'
  • less restriction leading to mechanical shock
  • hydraulic noise ( turbulence)
  • Mechanical Noise ( butting / rubbing)etc
This paper describes a systematic approach to cushioning in hydraulic cylinders.
  • design aspects
  • study of chamber pressure, cushion stroke & velocity
  • pressure and stroke sensing in cushion zone
  • noise aspects
  • Cushioning hydraulic cylinders (small an big ) range from connected load of tiny buckets to huge mass.
Hydraulic Cylinder Cushioning - The problem definition
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In the simplest of the schematics, the problem can be represented in the following diagram.
P  -  Pressure ( due to load)
Q  -  Pump flow into cylinder
A  -  Port Cover (Cap)
B  -  Port on Head End Cover
af  -  Full area ( Piston)
ac  -  Annular Area (Piston area-rod area)
W  -  Connected Weight
WR - Weight of rod + piston
V  -  Velocity of load / rod ( initial)
X  -  Piston of rod
When the mass comes to a halt at the bottom of cylinder or head side, an impact jolts the cylinder i.e., impact = m. v / D t - short duration when the initial velocity terminates to zero.
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Looking at the x vs. t diagram, load moving with velocity is D x / D t and becomes zero at end butting point. The problem is to reduce this velocity D x / D t to some acceptable level so that end butting impact is reduced. This is called as "cushioning in "hydraulic cylinders" or sometimes referred as "softening the blow".

A small part of thestroke towards the end is used to create opposing force in the back area of the piston and this stroke is called as "cushion length / cushion stroke".

The time to reduced the velocity and butting is called "cushion time".

The trapped area and cushion length called as "cushion chamber / volume".

Applications

Invariably in mobile and industrial machines using hydraulic cylinders to actuate the loads / masses it is inevitable that end butting of piston will take place. Some examples are shown below.
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The end butting shock explained earlier has effect not only on the machine but also on the operator comfort.

The shock has to be absorbed and the load needs to be braked to reduce to velocity and hence to reduce the impact.

Creation of the opposing force towards the end
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in the normal working range of the rod the cushion length ls1 and ls2 do not enter the covers thus the oil freely flows from service ports A&B to chambers C1 & C2

When Ls1 enters ds1 or Ls2 enters ds2 then the oil between piston and covers get locked up and will pass through clearance thus creating throttling for the chamber oil. This throttling produces back pressure hence opposing force (back pressure x area)

Kinetic Energy E = ½ mv2
M - Moving mass
V - Velocity

Total Energy
E total = ½ mv2 + mgls sin a 10 - 3
a = Actuator orientation
g = 9.81 m / sec2
ls = length in mm
m = mass in kg
v = m/sec

Substituting and equating the above and assuming that vp reduce 0.1 vi the maximum pressure in the chamber at impact is given by :

Dpmax = AP2 K vi2 3x10-6/A02
A0 = Orifice Area
K = Orifice Constant

Inference:

Orifice diameter is easy to calculate but has highest pressure at impact and does not provide restriction after the impact (velocity drastically reduces ). This leads to hydraulic shock with damped oscillation.

- Combination of annular and orifice generally provide a better effect.

Multiple Orifices
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Where enough cushion length LB available, than multiple orifice combinations provide increased pressure drop cushion advances.

As plunger into the bore, the orifices cut out one by one thus increasing the restriction for the reduced flow due to decreasing pressure.

Stepped Sphere

Similar to multiple orifices stepped sphere produces annular restrictions a1 a2 & a3 length L1, L2, & L3. Generally this approach is adopted where sufficient cushion stroke and volume of oil available so that spread out restrictions are possible.
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Fine tuning annular restriction:
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While the annular restriction in terms of annular clearance and the cushion travel length, two / three notches on the plunger / bush produces a variable restriction.

The notches control the area and reduce the velocity against time & also produce hydrostatic balancing effect to float the plunger / bush
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H = tan a x lB
W = 2 r Sin b / 2
H = w / 2 tan b/4
Area = Pr2b/360 - W (r - h)/2
= @ 2 / 3 w x h

Basic Energy Equation
1 Annular Restriction
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Equating energies
Etotal = Wmech + W1
= WF + W2 + W3
Wmech = Fmech x lb
Wmech = Mechanical Work Done
Fmech = Mechanical Force
Lb = Distance Traveled
W1 = ½ x F1x1b
( Pressure drops to zero during stroke of lb)
The above applied force is equated by the following:
WF +F2.Lb+WF+ W2+W3
1.18 m lb 3 Dm3 /
(CRad)3 tt
WF = Friction Force
F2 = Opposing force due to back pressure
W3 = Hydraulic work done
Crad = Radial clearance
m = Viscosity
Tt = Cushion Time
F1 = Drive Force
V1 = Initial Velocity
Dm = Cushion Diameter
Equating above equations we get
Crad = lb.Dm3 (1.18m / tt / 0.5 mv2) ½ + ( Fmech + 0.5 F1- F2) Lb - WF

In the above, the acceleration and compressibility of fluid not considered.
The cushion power Wpl
Wpl = 1.18 mlb3Dm3/
Crad3tt Cushion power is directly proportional to viscosity cube of length & diameter and inversely proportional to cube of clearance ( radius) & cushion time

( lb/ CRad )3 play an important role

Inference :
  • Increasing cushion length and decreasing clearance provide cubic increase in cushion power ( lb / CRad )3
  • Cushion power is proportional to velocity ( lb / tt)
  • For defined parameters cushion time is inversely proportional to (CRad)3
II Orifice restriction (assuming zero friction and no spring action)
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Newtons law of motion is sufficient to equate force and acceleration i.e.,
F = m . dvp /dt
VP = Piston Velocity
DP = Chamber Pressure
AP = Piston area for DP
For an orifice & oil, the head loss is given by DP @ K v02 3 x 10-6
As plunger travels, height and depending a tapers to zero. Thus in addition to annular restriction the notch produces variable area hence orifice effect.

Cushion chamber pressure decreases as the plunger is completing its travel, also the velocity decreases. Hence when piston hits the cover, it would have reduced the velocity to lower value hence the impact magnitude gets reduced. The slope of the stroke curve at which it gets flattened decides the effect on cylinder and connected structure. If the structure is on wheels / track then the machine gets blow ( hammer type).
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Excavator boom, arm bucket up operation

Example of end blow :

Figures shows a typical excavator when operated to lift up the boom, arm and bucket to extreme condition - butting.

The moving mass ( boom, arm & bucket structure) ultimately pull the boom rod fully up and if not cushioned will hit the cover hence creates jolting force at the rod end joint. This acts on machine, hence the machine hops a down. This results in

- structural damage to cover and
- operator discomfort
Extreme case of non-cushioning, heavy moving mass with high velocity may lead to toppling / capsizing / cover coming off / shearing off.

The Cushioning length and shaping of velocity curve by proper design of restriction play an important role, considering component, machine, operator safety and comfort.

Cushion Stroke Sensing :

Studying the behavior of stroke vs time during the plunger travel into the bush / cover provide the state of -

- Position x - (sense)
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- velocity & (x/t) = v - (compute)
- acceleration of the connected load v /t ( compute)
A - Plunger / bush enters the bore
B - Indicates unrestricted movements = butting load
C - Indicates acceptable restriction - smooth end
D - Indicates too much restriction leading to heavy Reduction of velocity at entry.
OAB - is characterized as " mechanical shock" all Velocity reduction / changes at B
OAD - is characterized as "hydraulic shock" max. Velocity change at A
OAC - is characterized as "smooth change" very Small change in velocity at A (x / t) and C (x /t)

Cushion Pressure Sensing :

When the cushion / plunger bush enters the bore thus providing restrictions for the locked up oil in the cushion chamber then the chamber pressure increase
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The typical characteristics of cushion chamber pressure during the cushion period are shown above.

Curve ABC :
Indicates a kick up pressure spike at the Entry of the plunger / bush and slow discharge with long cushioning time. This is termed as hydraulic shock. If the vehicle gets into dynamic oscillation, the discharge will also have oscillation. This is not comfortable to operator. Also the spike pressure may be very high (approx. 1000 Kfg /cm2 in big equipments / masses ) which is detrimental to structure as well as sealing elements.

Heavy restriction - close working clearance and longer cushion length have strong influence on these characteristics.

Curve ADE :
This indicates a pressure peak not a spike. Also lesser discharge time. Entry chamfer / radius annular restriction and cushioning length play an important role in generating these characteristics.

If the pressure peak is within the design limits, operator comfortable and vehicle does not experience shock / joint then cushion performance may be acceptable. Laboratory and field trials necessary to check cushion performance.

AFGH :
This indicates multiple pressure peaks due to changed annular restrictions / cutting out chain of orifices in the cushion plunger.

While handling very large masses and sizes in terms of bore and rod it become necessary to provide sufficient area for chamber flow in the beginning and cut off the areas as the plunger travels in the cushion bore.

Curve APQ.
In this chamber pressure raises slowly to steady value and discharges slowly to minimum. Looks like an ideal cushioning. This is a case of pure orifice restriction. This is characterized as parabolic (x vs. t ) cure will be a parabola and not linear. This indicates orifice flow (DQ u DP) 1 /2 or P u v2 u x2).

While entry and annular restriction provide the coarse part of cushioning, the notch and orifice will provide finer aspects of cushioning.

Curve ARS :
Short small duration and small pressure indicates no restriction for the cushion flow. In this case the entire momentum gained by the connected mass bangs the cover ad hence equipment. This is a jolt and heavy mechanical shock.

Cushion Performance Evaluation Tests & Tests Setups

Cushion performance is evaluated by quality of cushion in terms of

- smoothness
- softness
- no shake up / shocks
- comfortness

Following table indicates parameters which control cushion performance and measurable to evaluate performance.

Inputs Measurables
- Annular Clearance - Position
- Entry chamfer / radius - Drive Pressure
- North width / angle - Chamber Pressure
- Orifice - Noise


Test Setups ( any of them or combination )

Test 1 : Free Cycling with drive pressure
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Connect free cycling - under no load (rod free end) hydraulic power supply to cylinder (nominal flow and relief pressure adjustable in the range 50 .320 Kgf cm2)

Pressure transducers P1 and P2 pick up time histories of the pressures.

Position transducer will pick up the position of the plunger. Velocity and deceleration are computed from xvs.t.

Test 2 : Crank Mass System
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Small cylinders operating smaller masses- best result can be achieved by testing using cranking mass. Typical example is a mini excavator boom lifting once the bucket finishes digging and picks up the dug then boom is lifted up (fully) followed by a swing and a dump. During this, the cylinder cushion absorbs the mass energy to reduce velocity.

LVDT and pressure transducers pick up the waveforms to study the state of position and pressure.

Test 3 : Connected Mass ( Axial)
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Horizontal Set -up

For horizontal setup the mass is connected to rod ( rigid and very less / no friction). This is almost like a pure mass system.

In this, we can study the performance of cushion under.
A Influence of mass only ( by cutting of pump supply after achieving motion).
B Influence of mass and drive pressure by retaining pump supply.

Vertical setup can be in low mass - small cylinder applications. Horizontal setup is used in case of heavy large cylinder with heavy connected mass ( on trolley).

Test4 : Actual measurements on equipment installation / machine ( final approval )
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A 100 mm or sufficient length spring return LVDT and 1000 Kgf / cm2 pressure transducer on the drive line and 100 Kgf / cm2 pressure transducer on the cushion chamber pressure port (if available) sufficient to assess the cushion performance.

Some aspects on noise ( during cushion action)

Since the cushioning achieved by narrow working clearance and small orifices, following are the potential areas which can result in noise-

- Plunger / bush entry mechanical butting with bore (metallic noise)
- Plunger / bush travel inside bore - friction noise ( squeeching, scratching)
- Annular clearance / orifice (turbulence - hissing, in extreme case whistling)
- Sudden stoppage - mechanical butting ( bang noise)
- On retraction sometimes the oil starvation leads to cavitation noise ( Vacuum, bottle opening, marble, pebble etc.)

Engineers Stethoscope will help hear the noise at the location and smoothness of cylinder cushion action).

Some aspects of heart during cushioning

Hydraulic cushioning is accomplished by creating restriction for the flow, viscosity oil plays an important role in resistance hence pressure. Viscosity decreases with increased temperature, hence the chamber pressure thus reducing cushioning. Checking performance at 300C, 500C and 800C of oil ensure cushion effectiveness in he working temperature rang.

Cylinder Retraction after cushioning :

Since close working clearance produced cushioning, it is important that a quick start back should take place in the cylinder. Many designs incorporate a free flow check value on the return side so that quick flow of oil ensures quick start back action A right return leads to jerky start.

A good cushion and retraction stroke vs time is in the figure

A - Start of cushioning
B - End of cushioning
C - Start of retraction
D - End of Cushion length travel

No change in the slope of x vs. t during the retraction in the region CDE indicate retraction without jerk

Cushion Durability Tests :
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Since cushioning is a accomplished with very small working clearance creating high pressure in the cushion chamber both wear and strength aspect are to be assessed in a laboratory durability test bench.

Critical areas for assessment during durability test

- Wear (scoring ) rubbing / scratching
- Bulging bush / tube
- Seals, Piston & Cover
- Flange bolts and piston nut integrity and strength

Summery :
1. Cushioning in hydraulic cylinder explained with physics and mathematical model.
2. The controlling input parameters (design) and measurable output parameters ( from performance consideration identified and sensitizes explained.
3. Stroke and pressure waveforms use effectively to verify design output
4. Test and setup described.
5. Heat and noise aspects discussed

Conclusion

A systematic approach presented to design cushioning for hydraulic cylinders.

Acknowledgments

Thanks are due to Mr. K N Padmanabha, Vice President- Head of Construction Equipment Hydraulic Business and his team members who have systematically approached the cushioning problem and provided an effective solution.

Thanks are also due to Mr. Seethapathy, President Managing Director for permission present this work. Last But not the least Ms. E K Sandhya deserves sincere thanks for excellent typing of the script.