Document kDeb5k1KokNQGDK96aRN9d5wB

iwy ep LIA2A815 LEAD INDUSTRIES ASSOCIATION, INC. SSS MADIION AVCNUK Nrw YOAK IT. N. V. AL. March 30, 1962 SUBJECT: LEAP ASBESTOS AJfrrvib r a t io h Ea s To Meabsrs of the heed Industrie! Association, Zac: . Attached la a preliminary report 00 the evaluation of lead asbestos antivibrutlon pad* resulting froa research conducted through the Expanded Research Program nf L.I.A. As explained In the Introduction, the data la this report are In no sense to he considered final and definitive, since further research Is underway to verify the data and to clear up acoe of the apparent saooalies, as veil as to support the data vlth field tests 00 actual Installations. its have therefore printed only a Halted number of this prellnlnary report, vfclch ve axe eendlng to all members to be used vlth discretion should they have inquiries on this subject. We also plan to use It carefully In dealing vlth engineers vbo are contemplating the use of these pads, pending the final report, which ve hope will he available around the end of this year. Very truly yours. RLZ:ak fiie. Executive Vice President vmfm.,*. yysngv<j|'ro LIA24803 LEAD INDUSTRIES ASSOCIATION, Inc 211 Am mi N.- Yw* 17. N. Y. PrilliUirr Report EVALUATIO* or LEAD-ASBESTOS AIT I V IB SAT I 07 PADS This preliminary report. Issued March 15, 1962, covers the Initial research bjr the Expanded Research Program of lead Industries Association, Inc., and vill be followed by supplementary data In the near future. S M flin in 'm iM a liia irifili a la in ro-'-r "new N 1754.01 LIA2A80A *s"^.. --- EVALUATION OP LEAD-ASBESTOS A.TU-VIBRATION PADS _______ Preliminary Report 2 iKTRopumoH Lead, alone and in combination with asbestos, has been used to reduce the transmission of vibration In structures since before World War I. Lead and asbestos pads one Inch thick underlie the foundations of the Waldorf-Astoria Hotel In Hew York, for example. Similar pads have been used to support heavy eiachlnery such as cooling towers and printing presses. Lead has been used as the bedding surface of antl-vlbratlon pipe bangers. Because of accumulated experience with this use of lead, many of its antiribratloo properties are veil known: its extreme durability, practical methods of manufacture for pads, best methods of installation, etc. Recent studies of lead-ashestoe pads were undertaken because of two current engineering trends. First, the need for taxlrsas utilization of material in today's competitive engineering picture and, second, the desire of users and manufacturers of 6uch pads to achieve standardization. Despite Intensive study, partly covered by this preliminary report, the maximum limits of operation for lead-asbestos pads are not veil known. In terns of static loading, for example, typic*l practice of the 1920'3 called for 200 to 300 psl while tee accepted norm for the same pads today ic 600 to 800 psl or hitfier. Short-time static teste have net revealed any failure cf the pad for loads up to 15,000 psl. Regarding standardization, it is hoped that continuing studies will make It possible to appraise a vibration problem by simple, routine calculation methods and specify a standard mounting -- perhaps by "catalog nuaber"-vhlch will meet the requirements. In response to m-re extensive use of the pads and the growing demand for engineering data about them, the Lead Industries Association, Inc., has prepared thlo preliminary report. It Is drawn largely from a laboratory project undertaken through the Expanded Research Program cf the LIA and carried out by Lessells and Associates cf Boston. This project has raised new questions shout pad behavior which require additional work. A second generation of research Is now in pregreso to attempt to check the effectiveness of the pads at various loadings and conditions and to answer these new qj'-stlonc. In addition, the Association's Expanded Research Program hopes to cxcplrte field studies cf the pods in building foundations and machinery r.euntc eo that performance of the pads in practice will be known and the effects, of foundation, soli structure, and ether variables may be apparent. A problem cctncn to early studies cf vibration In structures has been the accurate determination of the frequency-intensity profile of the vibration encountered. Reliable measuring equipment is bulky and ccrplex. Sene c  m*0^JtSSStm V - J* fcfrrtf^T-^lwr m tmw* LIA24805 otherwise meticulous ctudles have employed simpler Instrument# and are thua I based on Intensity measurements of doubtful worth. In the project reported here, an effort '/as made to get hl^i reliability In the data. The difficulties in arriving at a suitable experimental set-up, and the further difficulty of accurately measuring the small values of the variables any have led to appreciable error In sene of the findings. Inspection of the data would lead to the conclusion that the results reported for the higher static loadings (100 and 600 pol) are probably better than thocc for the lower loadings. Furthermore, the teits reported here Intentionally simulated the simplest possible arrangement of the pad, structure, and vibration path. Any practical a;plication of the pads would probably Involve a mssber of com plicating factors. For these reasons the UA does not represent the data contained in this report to be performance data for the pads. It Is, rather, the best available Indicative data on the general behaviour of lead-asbestos pads. While Lead Industries Association, Inc., cannot, st this time, noke firm recemendations for use cf the pads, it will be pleased to work with engineers and architects Interested In using the pods and will, as always, be glad to make available its experience and test Jud<pent In the solution of specific problems. SESCRIPHOK OF THE PkTC,, THE TEST., A.TO TH5 RSXUS Hie pads tested In this project were lcod-asbestcs anti-vibration pods of typical construction measuring 10 x 10". The nets leal one-inch thickness of the pod is cade up of a top layer of 1/6" lead, an upper layer of 3/6" asbestos roll fire felt, a middle layer cf 12-guage galvanized steel, a lower layer of 3/6" asbestos as above, and a bottom layer cf 1/6" lead. The pads were sub jected to 200 pal before completely sealing the edges by lead burning. Actual thickness of the pads before test was 1 3/32". Static Test A short-tloe static loading test applied 15,000 psl to the pad without any evidence of failure. The "set" taken by the pad at 2,000 psl (short-time) was 3/16" -- final thickness being 29/32". Frcn the static loading data (page 5) It can be seen that a typical modulus of elasticity for the pod at 800 pal static It 8,900 psl. (This modulus la computed from the first upload data using the nominal pad thickness of one Inch.) Pyr.aale Tests Performance of the pads was measured by bolting them between billets to apply the prescribed static load and applying a single frequency vibratory force of up to **,000 pounds. The full frequency range (5 to 100 epa) was 1W1 '. iwjp^gi >j|(ijgw!m alga * . <y.<ilK LI A 24806 used by making one run at each of a number of frequenclea. Tvo pads were employed In each measurement. Agreement between pads vas good at high static loadings and rather poor at lov static loadings. Generalized behavior data Is presented In the four sets of curves ce pages 6 to 9. the key values on which these plots are based are presented In Table I. TABUS Z MODULI AND NATURAL FREQUENCIES OF LEAD-ASBESTOS ANTI -VIBRATION PADS for Nominal l" Pads Ps Static Losd pal 100 200 LOO 600 K Calculated StlffneEs lb.!u In. T 0e023 0.097 0.122 0.196 fn Calculated K&turatl Frequency cpa L3 69 56 50 E Calculated Dynamic Modulus 22,760 97,000 122,400 196,300 Notes: K value Is per square Inch of pad area. Approximate values of natural frequency of stacks of pads nay be obtained by dividing fn, aa given, by the square root of the number of pads in the stack. * * iw m. jf* Jite, ILMK LIA 2A 807 page 5 ?iQWU5<mA,*w*imw 1. JLMU r* PW iMjUMMUPMe WI - nUi iiiM^iiMri'^iiittfrirni ^ tvifurn^- m LIA 24 808 Ufc: 6 o u t p u t in p u t r at io o f f o r c e./ # h mM< 6 \ L IA 24809 W J "i" ^srvyi,! "' Iiw nr mm ( /o u t pu t in p u t r at io o f f o r c e. TRANSMISSiElUTY < v el o c it y o r a mp l it u d e 1 o f v ibr at io n nmm qwcpiw # I dm o Artfaiiw-JMh 0 FORCING FREQUENCY-CPS i H.J'llfH i iu > ii i WFggr^|wywg^l*W^^r^WiW*lwa*qg lIA 24 812 Md FRECj'HalCY CONSIDERATIONS 10 Moot vibration occurlng In building structures la unpredictable and complex. It la almost impossible to say beforehand vhat frequencies and Intensities will be encountered. The profile of Intensity vs. frequency for vibration caused by a railroad train has been found. In cue case, to bet Frequency Band, cpa Intensity, db (arbitrary reference) 20-75 3 75-150 16 150-300 300-600 600-1200 18 14 5 Note that the most Intense vibration la In the 75 to 600 cycles per secoed frequency range. For standard lead-asbestos pads 1" thick loaded to 600 psl tests Indicated about 85 percent Isolation. It would appear to be a simpler case vfaen the annoying vibration la caused by a large fan, for example, of known shaft speed. It has been suggested that such a shaft speed can be divided by 60 (seconds per minute) to arrive at the fundamental vibration frequency. Tests of such equipment have invariably shewn this to be false. A better approximation is to divide by 60 and multiply by the nusber of blades In the fan (or vanes In the pump, independent compression strokes of the compressor x ?, etc.) This will provide an Indication of the lowest frequency of disturbance for the source. Integral multiples of this (x 2, x 3. etc.) will also be important. In addition, there will always be high frequency noise. So long as there Is no Important lov-frequency resonance In the suppurtlng structure, lead asbestos pads have tested 75 to 95 percent effective In Isolating such vibration. TYPICAL OSES OF THESE PAT* Example 1 -- Building Isolation Lead encased pads have been used for many years for Isolation of buildings from objectionable vibration as might be present near railroad or subway rlghts-of-way or heavy traffic. For this example, assume that the vibration Is In the range from 150 to 300 cps, and that the pads are loaded to a static stress of 800 psl. If two lead asbestos pads, each 1" thick are stacked, the expected transmlsslblltty may be obtained from figure 5 (page 9) This plot shows that the percentage of vibration that will be transmitted under these conditions will range from 4.5 percent to 1.3 percent over the frequency range. *T*rr w m LIA 2A 813 9/ -*" -1 '"* ----- , ,-,-tV I ' - -------- Jt--` " 11 Example 2 -- Machine Isolation These pads can be equally effective for Isolating vibrating machinery from eurroundlngs. Conalder the case of large fan type Movers. One of the objectionable frequcnctca In this case would be associated with the nunher of blades 1c the fan and the shaft speed. Assume 12 blades and a speed of 1,200 rpr*. 1200 60 x 12 2U0 cps* For lead-asbestos pad loaded to 200 pal the tronsmlsslblllty will be 8 percent. (See figure 3, page Hie tronsmlsslblllty for hlgier harmonics of this frequency will be lower than this value and can be determined In the sane manner. Example 3 -- Broad Spectrum Isolation There will be applications where the Impressed vibrations cover a wide range of frequencies with the lower values being below the range of application for these pads. For these lever frequencies, coll spring Isolators must be used. These spring isolators will, however, transmit higher frequencies as longitudinal vaves along the veund helix. If In these coses, the springs are mounted on lead asbestos pads, a broad band Isolator will result. ATPEHDDt The LIA again points out that the data contained In this report should not be used for desi/pi. This appendix has been added to explain the moat coonon source of discrepancy between the Indicated behavior of lead-asbeatos anti-vibration pads and the actual behavior In tone cases. In many applications cf vibration Isolators, the structure which supports the system Is not rigid. An example of such a system Is an air conditioning compressor located In a building on a floor which Is above grade. If, for ouch systems, the floor stiffness U regarded os a massless spring, the chart In Figure 6 deccrlbec the response. This curve shews that for 6uch systems both the frequency ratio (ratio of disturbing frequency to the natural frequency of the pad) and the ratio ~f stiffness of the lcolator pad (K-j ) to the foundation or floor otirfD-ss (Kj > are Important. For a given foundation atlffnesa, therefore, a low value of K, or pad stiffness, Is desirable. 5 Any unbalance of the shaft will, of course, cause vibration at 1200/60 or 20 cps. In practice, machinery In reasonably good operating condition does not 6hov any obJcctUnnble tendency to vibrate at this low frequency. w '> V \\ Llki+*H + Ai<7frn'dti Wi'ift'fti'ftT" 12 < TRANSMISSIBIUTT VERSUS FREQUENCE RATIO FOR SEVERAL VALUE! Of THE RATIO Of MOUNT STIFFNESS TO FOUNDATION STIFFNESS Fleure 6 mij/v